Mesozoic (Upper Jurassic-Lower Cretaceous) deep gas reservoir ...

E&P NOTEMesozoic (Upper Jurassic–LowerCretaceous) deep gas reservoirplay, central and easternGulf coastal plainErnest A. Mancini, Peng Li, Donald A. Goddard,Victor Ramirez, and Suhas C. TalukdarABSTRACTThe Mesozoic (Upper Jurassic–Lower Cretaceous) deeply buriedgas reservoir play in the central and eastern Gulf coastal plain ofthe United States has high potential for significant gas resources.Sequence-stratigraphic study, petroleum system analysis, and resourceassessment were used to characterize this developing playand to identify areas in the North Louisiana and Mississippi Interiorsalt basins with potential for deeply buried gas reservoirs.These reservoir facies accumulated in Upper Jurassic to Lower CretaceousNorphlet, Haynesville, Cotton Valley, and Hosston continental,coastal, and marine siliciclastic environments and Smackoverand Sligo nearshore marine shelf, ramp, and reef carbonate environments.These Mesozoic strata are associated with transgressiveand regressive systems tracts. In the North Louisiana salt basin,the estimate of secondary, nonassociated thermogenic gas generatedfrom thermal cracking of oil to gas in the Upper Jurassic Smackoversource rocks from depths below 3658 m (12,000 ft) is 4800 tcf of gasas determined using software applications. Assuming a gas expulsion,migration, and trapping efficiency of 2–3%, 96–144 tcf of gas ispotentially available in this basin. With some 29 tcf of gas beingproduced from the North Louisiana salt basin, 67–115 tcf of inplacegas remains. Assuming a gas recovery factor of 65%, 44–75 tcfof gas is potentially recoverable. The expelled thermogenic gas migratedlaterally and vertically from the southern part of this basin tothe updip northern part into shallower reservoirs to depths of up to610 m (2000 ft).Copyright #2008. The American Association of Petroleum Geologists. All rights reserved.Manuscript received July 26, 2007; provisional acceptance September 28, 2007; revised manuscriptreceived October 19, 2007; final acceptance November 12, 2007.DOI:10.1306/11120707084AUTHORSErnest A. Mancini Center for SedimentaryBasin Studies and Department of GeologicalSciences, University of Alabama,Tuscaloosa, Alabama 35487;emancini@geo.ua.eduErnest A. Mancini is a Distinguished ResearchProfessor in petroleum geology and stratigraphyin the Department of Geological Sciencesand director of the Center for SedimentaryBasin Studies at the University of Alabama. Hisresearch focus is on sequence stratigraphy,sedimentary basin analysis, petroleum systemstudies, and reservoir characterization andmodeling.Peng Li Arkansas Geological Survey, LittleRock, Arkansas 72204; peng.li@arkansas.govPeng Li obtained his Ph.D. in geology fromthe University of Alabama (2007) and hasrecently joined the Arkansas Geological Surveyas a senior fossil fuel geologist. His researchaccomplishments include petroleum systemanalysis, basin modeling, and resource assessment.Currently, his research focuses onunconventional gas reservoir studies in theArkoma Basin.Donald A. Goddard Center for EnergyStudies, Louisiana State University, Energy,Coast & Environment Building, Baton Rouge,Louisiana 70803; dgodda1@lsu.eduDonald A. Goddard is an associate professorand petroleum researcher at Louisiana StateUniversity’s Center for Energy Studies (CES). Histasks at CES involve integrated field studiesin mature petroleum regions of the Gulf Coastas well as identifying and transferring upstreamtechnologies to independent operatorsthrough the programs of the PetroleumTechnology Transfer Council.Victor Ramirez Center for SedimentaryBasin Studies and Department of GeologicalSciences, University of Alabama, Tuscaloosa,Alabama 35487; present address: ECOPETROL,Bogotá, Colombia;victor.ramirez@ecopetrol.com.coVictor O. Ramirez is an exploration geologist forEcopetrol S.A. in Colombia. He obtained a B.A.degree in geology from the National Universityof Colombia in 1993 and an M.S. degree fromAAPG Bulletin, v. 92, no. 3 (March 2008), pp. 283–308 283

the University of Alabama in 2007. His workand research focus on sequence and seismicstratigraphy and petroleum systems as appliedto hydrocarbon exploration in Colombia.Suhas C. Talukdar Baseline Resolution,Inc., 143 Vision Park Blvd., Shenandoah, Texas77384; stalukdar@baselinelabs.comSuhas C. Talukdar is a geologist and geochemistwith more than 35 years of professionalexperience in industry, research, andteaching. He received his Ph.D. in geologyfrom Rice University in 1973. His expertise is inpetroleum geochemistry, basin modeling,petroleum system analysis, and hydrocarboncharge assessment for plays, prospects, andintegrated basin studies. He is presently asenior geochemist with Baseline Resolution,Inc.ACKNOWLEDGEMENTSWe thank Platte River for the use of theBasinMod 1 1D and BasinView 1 softwareapplications, and we thank IES for the use ofPetroMod 1 software applications. We especiallythank Carl Dillistone and Vic Smart forour stimulating discussions over the past twodecades about the petroleum geology of theMesozoic strata of the onshore interior saltbasins. We are most appreciative of theassistance of Jim Donahoe in preparing thegraphics in this article. We thank the presidentof the Gulf Coast Association of GeologicalSocieties for permission to modify figures inour articles published in the 2006 and 2007Gulf Coast Association of Geological SocietiesTransactions volumes and the associate editorof Carbonates and Evaporites for permissionto modify a figure in our 2003 article publishedin the AAPG Bulletin for inclusion as figuresin this article. The manuscript reviews byKenneth J. Bird and John B. Curtis improvedthe quality of the article. This research wasfunded by the U.S. Department of Energy(DOE), Office of Fossil Energy through theNational Energy Technology Laboratory. However,any opinions, findings, conclusions, orrecommendations expressed herein are those ofthe authors and do not necessarily reflect theviews of the DOE.INTRODUCTIONThe interior salt basins and subbasins in the central and eastern Gulfcoastal plain of the United States (Figure 1) have been areas for oiland gas exploration and production for more than a century. Productionfrom fields in this region exceeds 5 billion bbl of oil and43 tcf of gas. In 1999, the U.S. Geological Survey, in its assessmentof the oil and gas provinces of the world by known petroleum volumes,ranked the North Louisiana and Mississippi Interior salt basinsin the top 8% of 406 world-class hydrocarbon basins (Klett et al.,1999). Much of the remaining hydrocarbon potential in these basinsis in deeply buried Mesozoic facies.This article presents results from two 5-yr basin analysis andpetroleum systems studies of the North Louisiana (Mancini et al.,2006a) and Mississippi Interior (Mancini et al., 1999) salt basinsand from a 3-yr petroleum resource assessment study of the NorthLouisiana and Mississippi Interior salt basins and Manila and Conecuhsubbasins (Mancini et al., 2006b). These projects were supportedby the U.S. Department of Energy (DOE).The purpose of this article is to use the research results fromthe DOE projects and publications on sedimentary basin studies,petroleum system analysis, and resource assessment of the interiorsalt basins and subbasins resulting from these projects to characterizea deep gas Mesozoic (Upper Jurassic–lower Cretaceous) reservoirplay in the central and eastern Gulf coastal plain.METHODSThe DOE studies included the construction of 11 cross sections consistingof 141 wells for the Louisiana part of the North Louisianasalt basin, 5 cross sections consisting of 48 wells for the MississippiInterior salt basin, and 5 cross sections consisting of 18 wells forthe Manila and Conecuh subbasins. Selected regional seismic lineswere studied in support of the preparation of the cross sections andthe petroleum system characterization and modeling. Burial-history,thermal-maturation, and hydrocarbon expulsion profiles were preparedfor the 207 wells studied using Platte River BasinMod 1 1Dsoftware. The information used for each of the wells in this softwareapplication included formation ages, formation tops, formationthicknesses, amount of sediment compaction, stratigraphic levels forunconformities and associated hiatuses, corrected bottom-hole temperatures,present-day heat-flow values, paleoheat-flow (geothermalgradients) values, thermal conductivities of rock units, measuredand original total organic carbon (TOC) contents, kerogen types,vitrinite reflectance and thermal alteration index values, and pyrolysisdata, including maximum temperature (T max ) and measuredand original hydrogen indices (HI). Geologic maps, including subsurfacestructure, isopach, formation lithology, erosional thickness,present-day heat flow, and lithospheric stretching b factor, wereprepared to assist with the petroleum system analysis and resource284 E&P Note

Figure 1. Map of the interior salt basins and subbasins and related structural features in the central and eastern Gulf coastal plainand the location of the cross sections prepared and key wells studied. Cross sections, constructed using seismic and well-log data, forthe North Louisiana salt basin are designated as AA 0 through DD 0 , and cross sections, constructed using well-log data, are designatedas EE 0 through GG 0 from west to east in the study area.assessment. A transient heat-flow model was used in thepetroleum system characterization and modeling. Hydrocarbonflow pathways were modeled based on thecross sections and representative seismic reflection sectionsusing IES PetroMod 1 2D software. Key parametersused in the sedimentary basin analysis and petroleumsystem modeling are listed in Appendices 1 and 2.Mancini et al. (2006a, b) include a discussion of theseparameters and the maps prepared.Geochemical analyses were performed by GeoChemLaboratories and by Baseline Resolution on Upper Jurassic(Oxfordian) Smackover source rocks (88 samples)and for other Jurassic and Cretaceous lime mudstoneand shale beds (54 samples) in the North Louisianasalt basin. Results from these geochemical analyses areincluded in the Mancini et al. (2006b) DOE report.Geochemical analyses for the Smackover lime mudstone(16 samples) in the Mississippi Interior salt basinwere published previously by Mancini et al. (2003),and geochemical analyses for the Smackover beds (26samples) for the Manila and Conecuh subbasins werereported previously by Claypool and Mancini (1989).In this article, the petroleum system as described byMagoon and Dow (1994) is used.The total hydrocarbons generated and the potentialamount of this resource that is classified as thermogenicgas was estimated for the North Louisiana and MississippiInterior salt basins and Manila and Conecuhsubbasins using the method of Schmoker (1994) andPlatte River BasinMod 1 1D and BasinView 1 softwareapplications by Mancini et al. (2006b). In this article,the estimate of the total thermogenic gas generated andexpelled resulting from the Platte River software applicationsfor the North Louisiana salt basin is used inthe characterization of the deep gas (Upper Jurassic–Lower Cretaceous) reservoir play. Peters et al. (2006)concluded that the use of the Schmoker (1994) methodmay result in an overestimation of hydrocarbon expulsionfactors and petroleum charge. This probably is thecase in this study where the estimate of the total thermogenicgas generated using the method of Schmoker(1994) is considerably higher than the estimate of thetotal thermogenic gas generated using the Platte Riversoftware applications. The results from the resourceMancini et al. 285

Figure 2. Sequence-stratigraphic framework for the central and eastern Gulf coastal plain illustrating the transgressive andregressive (T-R) sequences recognized and the major petroleum seal rocks identified (modified from Mancini et al., 2006b, c).286 E&P Note

Figure 3. Cross sections,constructed using seismicand well-log data, forthe North Louisiana saltbasin. (A) Southwest tonortheast cross section AA 0on the Sabine uplift intothe basin to the northshowing a series of saltpillows in the northern partof the basin. (B) Southwestto northeast crosssection BB 0 from the updipmarginoftheNorthLouisiana salt basin tothe southern limit of thebasin illustrating changesin stratal thickness and aseries of normal faults in thenorthern part of the basinand a salt diapir in thesouthern part. (C) Southwestto north cross sectionCC 0 on the Monroeuplift into the basin showingonlap of Upper Cretaceousstrata onto thisstructure and a salt diapirin the southern part of thebasin. See Figure 1 forthe location of the crosssections.assessment using the Schmoker (1994) method for thesebasins and subbasins and for the results using the softwareapplications for the Mississippi Interior salt basinand Manila and Conecuh subbasins are reported inMancini et al. (2006b, 2007).The deeply buried reservoir facies with gas potentialwere recognized from well-log, core, and petrographicstudies and were placed in a sequence-stratigraphicframework. The structure maps prepared for the NorthLouisiana and Mississippi Interior salt basins and Manilaand Conecuh subbasins were used to determinethe depths of the formations recognized as having reservoirpotential from the geologic studies and sequencestratigraphicanalysis. From the petroleum system characterizationand modeling, including the constructedvitrinite reflectance (R o ) profiles for these basins andsubbasins, those formations with facies buried to optimumdepths for thermogenic gas generation and preservation(R o value range of 1.3–4.0%) of 3658–6096 m(12,000–20,000 ft) in the North Louisiana salt basinand 5029–7620 m (16,500–25,000 ft) in the MississippiInterior salt basin were identified. The Manilaand Conecuh subbasins are oil prone (Mancini et al.,2006b). The cross sections prepared from the seismicMancini et al. 287

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and well-log data and the results from the hydrocarbonflow pathway modeling were used to define the formationshaving reservoir facies with thermogenic gaspotential. The results from the resource assessment providedanestimationofthesecondary,nonassociatedthermogenicgas potentially available. For mapping the optimumdepth for the formations and groups with facieshaving thermogenic gas potential in these basins andsubbasins, a reference depth level of 4572 m (15,000 ft)was selected. This reference level is intermediate betweenthe 3658-m (12,000-ft) depth in the North Louisianasalt basin and the 5630-m (18,470-ft) depth inthe Manila and Conecuh subbasins, which are recognizedas the critical depths for the generation of secondary,nonassociated thermogenic gas in this basinand these subbasins. The series of reference depth levelsdrawn through Louisiana, Mississippi, and Alabamaserves to delineate the areas in these basins and subbasinsthat have potential for reservoir facies containingthermogenic gas.GEOLOGIC SETTINGThe evolution of the onshore interior salt basins andsubbasins is related to the origin of the Gulf of Mexico(Wood and Walper, 1974). The Gulf of Mexico is a passivecontinental margin characterized by extensionalrift tectonics and wrench faulting (Pilger, 1981; Miller,1982; Salvador, 1987; Winker and Buffler, 1988). TheNorth Louisiana and Mississippi Interior salt basinsand Manila and Conecuh subbasins are major negativefeatures; and structural highs, such as the Sabine uplift,Monroe uplift, Wiggins arch, LaSalle arch, Choctawridge complex, and Conecuh ridge complex, act to separatethese basins and subbasins (Figure 1). Depositionwas associated with rifted margin tectonics and was aresult of basement cooling and subsidence that producedaccommodation space for sediment accumulation(Nunn, 1984; Sawyer et al., 1991). During the LateJurassic to the Early Cretaceous (Figure 2), the greatestaccommodation space was generated (Driskill et al.,1988).The tectonic evolution of the onshore interior saltbasins and subbasins has resulted in the formation ofstructural, stratigraphic, and combination structuralstratigraphictraps in the North Louisiana and MississippiInterior salt basins and Manila and Conecuh subbasins.Movement of the Jurassic Louann Salt has produced acomplex of salt-related structural features, includingpillows, diapirs, and extensional faults and half-grabensystems (Hughes, 1968; Lobao and Pilger, 1985). Thesefeatures form most of the petroleum traps in these basinsand subbasins; however, anticlinal structures associatedwith basement paleotopographic highs and stratigraphicupdip features associated with facies changesare also present (Mancini et al., 2003).The cross sections (Figure 3) prepared from seismicand well-log data for the North Louisiana salt basinshow the salt structures that have been described ascharacteristic of this basin by Lobao and Pilger (1985).Cross section AA 0 (Figure 3A) shows a series of saltpillows in the northern part of the basin, and salt diapirsare evident in the southern part of the basin in crosssections BB 0 (Figure 3B) and CC 0 (Figure 3C). Developmentof salt features, such as diapirs, appears moreadvanced in the southern part of the North Louisianasalt basin in comparison to the northern part of the basinwhere salt pillows are common. Movement of the JurassicLouann Salt commenced during the Late Jurassicin the southern part of the basin and was initiated inthe Early Cretaceous in the northern part of the basin(Lobao and Pilger, 1985). Cross section CC 0 also revealsonlap of Upper Cretaceous strata onto the Monroe uplift.Cross section BB 0 illustrates changes in stratal thicknessfrom the updip northern margin of the basin to thesouthern part of the basin (Figure 3B). This section alsoshows a series of normal faults from the northern margininto the basin.The cross sections, prepared from well-log data,for the eastern Gulf coastal plain show the characteristicsobserved in the stratigraphy of this area. Crosssection GG 0 in the Mississippi Interior salt basin revealschanges in stratal thickness and depth of burial from theupdip northern margin of this basin to the Wiggins arch(Figure 4A). Cross section HH 0 illustrates changes inthickness and depth of burial for the stratigraphic unitsFigure 4. Cross sections, constructed using well-log data, for the eastern Gulf coastal plain. (A) North to southwest cross section GG 0showing changes in stratal thickness and depth of burial from the updip margin of the Mississippi Interior salt basin to the northernedge of the Wiggins arch (modified from Mancini et al., 1999) and (B) north to southeast cross section HH 0 illustrating changes instratal thickness and depth of burial from the updip margin of the Conecuh subbasin to the center of the subbasin (modified fromMancini et al., 2005). See Figure 1 for the location of the cross sections.Mancini et al. 289

from the updip northern margin of the Conecuh subbasinto the center of this subbasin (Figure 4B).SEQUENCE STRATIGRAPHY STUDYMancini and Puckett (2005) and Mancini et al. (2006c)have reported 11 transgressive-regressive (T-R) stratigraphicsequences in the strata of the onshore interiorsalt basins and subbasins for the central and eastern Gulfcoastal plain (Figure 2). These T-R sequences includethree Upper Jurassic (Oxfordian) to Lower Cretaceous(Berriasian) sequences (Norphlet, Smackover, Haynesville,and Cotton Valley strata); three Lower Cretaceous(Valanginian to Albian) sequences (Hosston,Sligo, Pine Island, James, Bexar, Rodessa, Ferry Lake,Mooringsport and Rusk, Paluxy, Goodland, Andrew,Fredericksburg, lower Washita, and Dantzler strata);and five Upper Cretaceous (Cenomanian to Maastrichtian)sequences (upper Washita, Tuscaloosa, Eagle Ford,Eutaw, Austin, Mooreville, Demopolis, Taylor, Ripley,Prairie Bluff, and Navarro strata). The stratigraphic sequenceas characterized by Embry (2002) is used in thisarticle. Each sequence consists of a transgressive systemstract (deepening-upward section) and a regressivesystems tract (shallowing-upward section). The sequenceboundaries are defined by physical surfaces, such assubaerial unconformities, shoreface ravinement surfaces,transgressive surfaces, or surfaces of maximumregression, and a surface of maximum transgression separatesthe two systems tracts (Embry, 2002; Manciniand Puckett, 2005). These T-R sequences form theframework for the facies interpretation and reservoiridentification in this study. Emphasis is placed on theUpper Jurassic and Lower Cretaceous sequences becauseof their potential as thermogenic gas reservoirs.PETROLEUM SYSTEM ANALYSISPetroleum Source Rock IdentificationThree active petroleum source rocks have been recognizedin Upper Jurassic and Cretaceous strata of theGulf coastal plain and south Florida (Mancini et al.,2001a). Upper Jurassic (Oxfordian) Smackover limemudstone has been determined to be an effective regionalsource rock in the North Louisiana and MississippiInterior salt basins and Manila and Conecuhsubbasins (Oehler, 1984; Sassen et al., 1987; Claypooland Mancini, 1989; Mancini et al., 2003, 2005). UpperCretaceous (Cenomanian–Turonian) Tuscaloosa marineshale has been described as a source rock in Mississippi(Koons et al., 1974); however, these shale bedswere only effective as local petroleum source rocks inparts of the Mississippi Interior salt basin (Manciniet al., 2001b, 2005, 2006b; Lewan, 2002). Lower Cretaceous(Albian) Sunniland lime-mudstone beds havebeen reported as effective local source rocks in southFlorida (Palacas, 1978; Palacas et al., 1984). Additionally,Upper Jurassic (Tithonian) shale and carbonatebeds have been described as source rocks in Mexico(Mancini et al., 2001a), and Upper Jurassic (Tithonian)to Lower Cretaceous (Berriasian) Bossier shale bedshave been reported as source rocks in the East Texas saltbasin by Ridgley et al. (2006). The uppermost Jurassicand Lower Cretaceous lime mudstone and shale beds,particularly the Bossier shale, are possible source rocksin parts of the North Louisiana and Mississippi Interiorsalt basins given the proper organic facies andsufficient TOC contents for they have experienced therequired thermal conditions for thermogenic hydrocarbongeneration, according to Mancini et al. (2005,2006b).Lower Tertiary (Paleocene–Eocene) Midway, Wilcox,and Sparta shale beds have been recognized as sourcerocks in southern Louisiana by Sassen (1990), and Sassen(1990) stated that the Paleocene–Eocene Wilcox lignitebeds are possible source rocks in southwestern Mississippi.However, Mancini et al. (2005, 2006b) reportedthat Paleocene and Eocene shale and lignite bedshave not been subjected to favorable burial and thermalmaturationhistories required for petroleum generationin the North Louisiana and Mississippi Interior salt basinsand the Manila and Conecuh subbasins.Petroleum System Components and Play CharacteristicsThe various components of the Smackover petroleumsystem in the North Louisiana and Mississippi Interiorsalt basins and Manila and Conecuh subbasins werecharacterized in Mancini et al. (2003, 2006b, 2007).These components include the underburden, overburden,source, reservoir, and seal rocks of this petroleumsystem that are associated with petroleum traps.The characteristics of the underburden and overburdenstrata are a result of their rift-related geohistory.Underburden rocks consist of Paleozoic crystalline andsedimentary rocks (prerift); Upper Triassic to LowerJurassic alluvial and fluvial graben-fill redbeds of the290 E&P Note

Table 1. Comparison of the North Louisiana Salt Basin and Mississippi Interior Salt BasinSource Rock Characteristics(Smackover) North Louisiana Salt Basin Mississippi Interior Salt BasinThickness (m) 396 116Kerogen type Type IIS (microbial-amorphous, liptinite) Type IIS (microbial-amorphous, liptinite)TOC (wt. %) range 0.06–8.42 0.24–4.55TOC (wt. %) average 0.58 0.85Depth to generate oil, m (ft) 1829–2591 (6000–8500) 2438–3353 (8000–11,000)Depth to generate gas, m (ft) 3658–4877 (12,000–16,000) 5029–6401 (16,500–21,000)Basin ProperUpdipand MarginDowndipand CenterMonroeUpliftSabineUplift Updip and Margin Downdip and CenterMaturity (R o [%]) 0.8–1.3 1.3 to >2.6

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The original predominant kerogen types as observedthrough visual kerogen analysis are amorphous derivedfrom biodegradation of microbial matter and microbial(Table 1). Sassen et al. (1987), Claypool and Mancini(1989), and Mancini et al. (2003) also reported that themain kerogen types in the Smackover beds are microbialand microbial-derived amorphous. Smackover samplesrecord thermal alteration indices of 2–4 (R o =0.55–4%) (Sassen et al., 1987; Sassen and Moore, 1988; Claypooland Mancini, 1989).Hydrocarbon generation and maturation trends havebeen observed by Mancini et al. (2003, 2005, 2006b)from burial-history and thermal-maturation profilesprepared for wells in the North Louisiana and MississippiInterior salt basins and Manila and Conecuhsubbasins. The burial-history profiles reflect the availabilityof greater accommodation for sediment accumulationin these basins and subbasins during the LateJurassic to Early Cretaceous (Figure 5A). Strata associatedwith basement horst blocks, such as the Sabine,Monroe, and Wiggins uplifts, experienced significantuplift and erosion during the Early to Middle Jurassic,Early Cretaceous (Valanginian), and Late Cretaceous(Cenomanian) (Mancini et al., 1999, 2006a). Periodicuplift of these horst blocks, which have been interpretedas remnants of early Mesozoic crustal extensionand rifting in the Gulf of Mexico, are a result of the reactivationof bounding basement faults caused by tectonicforces (Miller, 1982; Sawyer et al., 1991; Zimmermanand Sassen, 1993; Adams, 2006). The burial-history profilesfor wells in the Monroe uplift area (Figure 5B) showevidence for three Upper Cretaceous uplift and erosionevents in the Cenomanian, Coniacian, and Campanian(Mancini et al., 2006a).In these basins and subbasins, the initiation of oiland associated gas generation is at an R o level of 0.55%,and thermogenic gas generation is at an R o level of 1.3%(Sassen and Moore, 1988). The production limit forwet-gas generation is at an R o level of 2.0%, and thermogenicdry gas generation ceases at an R o level of 4.0%(Nunn and Sassen, 1986). The generation of hydrocarbonsfrom the Smackover lime mudstone was initiatedat 1829–2591 m (6000–8500 ft) during the Early Cretaceousin the North Louisiana salt basin (Manciniet al., 2006a) (Figure 5C; Table 1), and the generationof hydrocarbons from the Smackover beds was initiatedat 2438–3353 m (8000–11,000 ft) during the EarlyCretaceous to Tertiary in the Mississippi Interior saltbasin (Mancini et al., 2003, 2005, 2006a) (Figure 5D;Table 1). Lewan (2002) reported that oil generationfrom Smackover source rocks was completed during theEarly Cretaceous in the onshore interior salt basins. Thegeneration of hydrocarbons was initiated at 2591–3353 m (8500–11,000 ft) from the Smackover limemudstone during the late Early to Late Cretaceous inthe Manila and Conecuh subbasins (Mancini et al.,2005, 2006b) (Figure 5F).In the North Louisiana and the Mississippi Interiorsalt basins, hydrocarbon expulsion from the Smackoversource rocks began first in the southern part of thesebasins during the Early Cretaceous to Tertiary (Manciniet al., 2003, 2005, 2006a) (Figure 5E; Table 1). Gas expulsionoccurred from the Smackover source beds inthe late Early Cretaceous through the Tertiary in thesouthern part of these basins (Mancini et al., 2006a)(Figure 5E). No thermogenic hydrocarbon expulsionoccurred from the Smackover lime mudstone in theimmediate vicinity of the Monroe uplift (Mancini et al.,2006a). In the Manila and Conecuh subbasins, oil expulsionfrom the Smackover source rocks commencedduring the Late Cretaceous (Mancini et al., 2005, 2006b).Hydrocarbon flow pathway modeling in this study(Figures 6–8) indicates that the main source of thesecondary, nonassociated thermogenic gas producedfrom reservoirs in the North Louisiana salt basin is generatedfrom the Smackover lime mudstone beds inthe southern part of the basin. Migration modelingbased on geologic models, constructed from well-logdata (Figures 6A, 7A) and seismic and well-log data(Figure 8A), show that initial oil expulsion commencedfrom the Smackover source rocks in the Early Cretaceousin the southern part of the North LouisianaFigure 5. Burial history, thermal maturation, and Smackover hydrocarbon expulsion profiles for key wells. (A) Burial-history profilefor the Continental No. 1 Bastrop well (API 1706900174) from the southern part of the North Louisiana salt basin. (B) Burial-historyprofile for the Carter No. C-1 Hope well (API 1706700061) from the Monroe uplift area. (C) Thermal-maturation profile for well API1706900174 from the southern part of the North Louisiana salt basin. (D) Thermal-maturation profile for the Corp. America No. 1Vasen well (API 2311100069) from the southern part of the Mississippi Interior salt basin (modified from Mancini et al., 2003).(E) Smackover hydrocarbon expulsion profile for well API 1706900174 from the southern part of the North Louisiana salt basin.(F) Thermal-maturation profile for the Humble No. 1 Huxford well (API 0105320007) from the central part of the Conecuh subbasin area(modified from Mancini et al., 2005). See Figure 1 and cross sections in Figures 4, 6A, and 7A for the location of the wells.Mancini et al. 293

Figure 6. Cross section,geologic model, and hydrocarbonflow pathwaymodeling for the NorthLouisiana salt basin.(A) North to south crosssection EE 0 , constructedusing well-log data, showingchanges in stratigraphyfrom the updip Northpart of the basin to thesouthern part of the basin(modified from Manciniet al., 2005). (B) Thermalmaturityand migrationmodeling at the initiationof oil expulsion duringthe Early Cretaceous inthe southern part of thebasin based on the geologicmodel constructedfrom cross section EE 0 .(C) Thermal-maturity andmigration modeling atpeak dry-gas expulsionduring the early Late Cretaceousin the southernpart of the basin. SeeFigure 1 for the locationof the cross section EE 0 .salt basin (Figures 6B, 7B, 8B), and that peak gas expulsionoccurred from the Smackover source beds inthe early Late Cretaceous in the southern part of thisbasin (Figures 6C, 8C). Migration of the expelled thermogenicgas into the Monroe uplift area occurred duringthe early Eocene (Figure 7C). Lewan (2002) also294 E&P Note

Figure 7. Cross section, geologic model,and hydrocarbon flow pathway modelingfor the North Louisiana salt basin.(A) North to south cross section FF 0 ,constructedusing well-log data, showingchanges in stratigraphy from the Monroeuplift into the basin (modified fromMancini et. al., 2006a). (B) Thermalmaturityand migration modeling at theinitiation of oil expulsion during theEarly Cretaceous in the southern part ofthe basin based on the geologic modelconstructed from cross section FF 0 .(C) Thermal-maturity and migrationmodeling at the time of dry-gas migrationinto the Monroe uplift area duringthe early Eocene from the southern part ofthe basin. See Figure 1 for the locationof the cross section FF 0 .Mancini et al. 295

Figure 8. Cross section, geologic model, and hydrocarbon flow pathway modeling for the North Louisiana salt basin. (A) Northwestto southeast cross section DD 0 , constructed using seismic and well-log data, showing changes in stratigraphy from the Sabine uplift tosouth of the Monroe uplift. (B) Thermal-maturity and migration modeling at the time of oil expulsion during the Early Cretaceous inthe southern part of the basin based on the geologic model constructed from cross section DD 0 . (C) Thermal-maturity and migrationmodeling at peak dry-gas expulsion during the early Late Cretaceous in the southern part of the basin. See Figure 1 for the location ofthe cross section DD 0 .296 E&P Note

concluded that thermogenic gas migrated into the areaof the Monroe uplift during the Eocene. The hydrocarbonflow pathway modeling demonstrates the importanceof vertical as well as lateral migration in thispetroleum system. Zimmerman and Sassen (1993) concludedthat vertical hydrocarbon migration along withlateral migration was fundamental to the productivityof the North Louisiana salt basin. The tectonic historyof the basin is critical to hydrocarbon migration in that ithas resulted in faulting (normal and wrench faultsassociated with fractures) to facilitate vertical migrationand stratigraphic and erosion surfaces to enhance lateralmigration (Zimmerman and Sassen, 1993).The chief difference in the geohistories of theNorth Louisiana and Mississippi Interior salt basins andManila and Conecuh subbasins was the increased heatflow that the strata in the North Louisiana salt basinexperienced in the Cretaceous probably as a result ofthe reactivation of uplift, igneous activity, and erosionassociated with the Monroe and Sabine features (Zimmermanand Sassen, 1993; Lewan, 2002). The strataassociated with the Jackson dome, which has been interpretedas an igneous intrusion by Kidwell (1951)and Dockery (1998), in the Mississippi Interior salt basinalso reflect an elevation in heat flow, but the effectsof this intrusion were more geographically limited.The results from this study support the conclusionby Zimmerman and Sassen (1993) and Lewan (2002)that the original source of the thermogenic gas producedfrom reservoirs in the Monroe uplift area is theSmackover Formation. However, differing opinions existas to the direct source of this thermogenic gas. Zimmermanand Sassen (1993) reported that the sourcewas Smackover beds associated with the Monroe uplift.Lewan (2002) concluded that the gas is a result of theconversion of oil to gas in reservoirs in downdip petroleumtraps during the Tertiary because of the high gas/oil ratios (GOR) inherent to this basin. The migration(hydrocarbon flow pathway) modeling performed inthis study has revealed that the source of the secondary,nonassociated thermogenic gas in the area of the Monroeuplift is probably the result of late thermal crackingof oil to gas in Smackover source beds south of theMonroe uplift and not in the immediate vicinity of thisfeature. The thermogenic gas originated in the southernpart of the basin and then migrated updip and verticallyduring the Tertiary into areas such as the Monroe uplift(Figure 7C). However, a combination of late migrationand vertical movement through overlying strata probablyresulted in a mixing of this thermogenic gas withgas that originated from the conversion of oil to gasin deeply buried Upper Jurassic and Lower Cretaceousreservoirs.Reservoir rocks in the North Louisiana and MississippiInterior salt basins and Manila and Conecuhsubbasins are Jurassic, Cretaceous, and Tertiary continental,coastal,nearshore,marine-shelf,and deep-marinesiliciclastics and nearshore marine shelf, ramp, and reefcarbonates (Mancini et al., 2003, 2006c). The lithology,interpreted depositional environment, porosity, and permeabilityfor these reservoirs are listed in Table 2.Petroleum seal rocks in the North Louisiana andMississippi Interior salt basins and Manila and Conecuhsubbasins are diverse (Figure 2). The seal rocks includeUpper Jurassic and Lower Cretaceous anhydrite andshale, Upper Cretaceous chalk and shale, and lowerTertiary shale (Mancini et al., 2003, 2006b).ESTIMATED TOTAL THERMOGENICGAS RESOURCESThe total thermogenic gas generated from the thermalcracking of oil to gas in the Smackover source rockswas estimated by Mancini et al. (2006b, 2007) for theNorth Louisiana and Mississippi Interior salt basinsand Manila and Conecuh subbasins using petroleumsystem software applications. This estimate does notinclude any thermogenic gas produced from the conversionof oil to gas in deeply buried reservoirs.For the Louisiana part of the North Louisianasalt basin, Mancini et al. (2006b, 2007) estimated thatthe total thermogenic gas generated for this basin is6400 tcf using the software applications (Figure 9A).Of this total gas resource, 75% or 4800 tcf of gas isestimated to be from late thermal cracking of oil to gasin the source rock and is categorized as secondary, nonassociatedgas based on using a depth limit of greaterthan 3658 m (12,000 ft), which approximates theupper depth limit for secondary thermogenic gas generationfor this basin (Mancini et al., 2006b, 2007).Schmoker (1994) reports that in most sedimentary basins,the expulsion, migration, and trapping efficiencyfor petroleum is a few percent, and Zimmerman (1999)used a hydrocarbon efficiency of 0.5% in his resourceevaluation of strata in south Louisiana. In this assessment,gas expulsion, migration, and trapping efficiencyof 2–3% was used to account for the hydrocarbonproduction to date from reservoirs in the North Louisianasalt basin; to accommodate the complex geohistoryof hydrocarbon generation, expulsion, migration,Mancini et al. 297

Table 2. Sedimentary and Petrophysical Characteristics of the Reservoirs in the Onshore Interior Salt Basins and Subbasins, Centraland Eastern Gulf Coastal PlainReservoir Lithology* Environment** Porosity (%) y Permeability (md)Wilcox SS FD 15–35 200–600Arkadelphia and Monroe Gas Rock CK NS, MS 5–25 1–500Selma and Jackson Gas Rock CK MS 5–18 1–500Nacatoch SS NS 20–28 200–2500Ozan SS NS 20–33 100–1000Tokio SS NS 20–35 200–450Eutaw SS T, NS, MS 13–39 1–5470Tuscaloosa SS FD, C, MS 25–30 200–2000Dantzler SS FD 25–30 50–150Fredericksburg SS, LS NS, MS, R 10–30 1–150Paluxy SS F, NS, MS 24–30 150–600Mooringsport SS, LS NS, MS, R 10–20 10–500Rodessa SS, LS NS, MS, R 10–26 10–650James LS NS, MS, R 10–15 6–100Pine Island SS NS 10–15 10–200Sligo SS, LS NS, MS, R 16–20 9–100Hosston SS FD, NS, DM 10–26 10–250Cotton Valley SS, LS FD, NS, R 9–18 1–300Haynesville SS, LS F, NS, R 9–16 50–400Smackover LS CR, R 11–22 1–100Norphlet SS E, F 3–20 1–890*Lithology: SS = sandstone, LS = limestone, CK = chalk.**Environment: E = eolian, F = fluvial, FD = fluvial-deltaic, C = coastal, T = tidal, NS = nearshore marine, MS = marine shelf, DM = deep marine, CR = carbonateramp, R = reef.y Reservoir porosity and permeability data are from files of the regulatory agencies and geological surveys (2006, personal communication) and the 2006 Quick lookhandbook onshore Louisiana petroleum producing formations by Goddard (2006, personal communication).and entrapment inherent to this interior salt basin resultingfrom the episodic reactivation of uplift and erosionof basement horst blocks, such as the Sabine andMonroe features; and to reflect the expected rock characteristics(depositional and diagenetic) and petrophysicalproperties (low porosity and permeability) of thedeeply buried Upper Jurassic and Cretaceous reservoirfacies. With a gas efficiency of 2–3%, 96–144 tcf of gasis potentially available in the North Louisiana salt basin.Some 29 tcf of gas has been produced from this basin(Table 3). Thus, 67–115 tcf of in-place gas remains.Using the software applications, a volume of 1280 tcfof gas, representing 20% of the total thermogenic gasgenerated and a gas expulsion efficiency of 20%, is estimatedto have been expelled (Figure 9B). Assuming agas recovery factor of 65%, 44–75 tcf of gas is potentiallyavailable for recovery.Based on the thermal maturity of the Smackoverlime mudstone in southern Arkansas as reported bySassen and Moore (1988), these source rocks have notreached a thermal-maturity level to generate secondary,nonassociated thermogenic gas. In addition, gas productionfrom Upper Jurassic and Cretaceous reservoirsin southern Arkansas is associated with oil production.The Smackover is at depths of 1524–3353 m (5000–11,000 ft) in this area (Moore and Druckman, 1981).Thus, the nonassociated gas generated in the Louisianapart of the North Louisiana salt basin apparently didnot migrate into southern Arkansas.For probabilistic estimation of the volume of secondary,nonassociated thermogenic gas available in theNorth Louisiana salt basin, the factor of hydrocarbonexpulsion, migration, and trapping efficiency was selectedto establish the P 90 , P 50 , and P 10 values for thepotential distribution of this resource. The highest levelof expulsion, migration, and trapping efficiency of 3%(144 tcf of gas) was used as the P 10 value, and the lowestvalue of expulsion, migration, and trapping efficiency of298 E&P Note

Figure 9. Maps showing the total gasgenerated and expelled for the NorthLouisiana salt basin using the softwareapplications (modified fromMancini et al., 2006b). (A) Map showingthe area of thermogenic gas generation.(B) map showing the area ofthermogenic gas expulsion. Note thelocation of the Continental No. 1 Bastropwell (API 1706900174) and the CarterNo. C-1 Hope well (API 1706700061)and the 3658-m (12,000-ft) contourfor reference for the basin. See Figure 1for the location of the wells and forthe latitude and longitude coordinatesfor the mapped area in the NorthLouisiana salt basin.Mancini et al. 299

300 E&P NoteTable 3. Oil and Gas Production from the North Louisiana and Mississippi Interior Salt Basins*North Louisiana Salt BasinMississippi Interior Salt BasinReservoirOil (bbl) Gas (mcf) GOR (scf/bbl) Reservoir Depth, m (ft) Oil (bbl) Gas (mcf) GOR (scf/bbl) Formation Depth, m (ft)TertiaryWilcox 228,200 89,342 392 610–2134 (2000–7000) 273,753,647 198,084,956 724 398–1177 (1307–3863)Upper CretaceousSelma and Jackson39,205,424 224,393,889 5724 921–1570 (3022–5150)Gas RockArkadelphia and44,038 7,452,904,183 169,238,026 610–762 (2000–2500)Monroe Gas RockNacatoch (Navarro) 758,374,196 4,431,274,239 5843 244–823 (800–2700)Ozan and Buckrange 265,037,353 1,007,534,243 3801 381–945 (1250–3100)(Taylor)Tokio and Blossom 128,817,273 1,718,406,462 13,340 823–945 (2700–3100)(Austin)Eutaw 301,449,711 1,754,506,272 5820 945–2448 (3100–8030)Tuscaloosa and Eagle 3,971,873 75,601,381 19,034 945–2819 (3100–9250)FordTuscaloosa 637,040,878 1,824,392,781 2598 1084–2602 (3558–8537)Lower CretaceousDantzler 783,201 72,450,931 92,506 943–2955 (3095–9695)Washita-Fredericksburg 56,943,318 255,821,157 4493 1446–2955 (4744–9695)Fredericksburg 1,643,190 34,409,159 20,940 914–1829 (3000–6000)Paluxy 6,206,760 88,408,279 14,244 762–2591 (2500–8500) 56,544,588 568,991,732 10,063 1730–3706 (5677–12,160)Mooringsport and 312,309 1,171,999 3753 914–1524 (3000–5000) 11,641,148 215,893,837 18,546 1677–4215 (5502–13,830)Ferry LakeRodessa 198,858,232 5,615,080,804 28,237 1250–1829 (4100–6000) 235,162,019 341,331,628 1451 1715–4088 (5628–13,413)James 12,409 2,869,335 231,230 1433–1829 (4700–6000) 902,320 80,356,905 89,056 2479–4237 (8133–13,900)Pine Island 8,745,072 545,229,418 62,347 1219–2134 (4000–7000) 543,856 676,027 1243 2479–4237 (8133–13,900)Sligo and Pettet 140,715,109 3,557,065,945 25,278 1524–2743 (5000–9000)Sligo 30,927,220 157,859,597 5104 2543–4478 (8343–14,692)Hosston 12,896,970 1,641,948,296 127,313 1524–3658 (5000–12,000) 54,887,990 995,065,210 18,129 2664–4640 (8740–15,223)

Upper JurassicCotton Valley 114,348,835 2,223,486,076 19,445 2195–4420 (7200–14,500) 106,461,276 146,163,240 1373 1437–5502 (4713–18,050)Haynesville 13,923,298 152,081,744 10,923 2957–3200 (9700–10,500) 6,421,491 349,786,844 54,471 1990–6367 (6528–20,890)Smackover 33,800,601 271,765,406 8040 3048–3810 (10,000–12,500) 522,979,535 4,069,721,819 7782 2038–7179 (6685–23,553)Norphlet 12,664,335 331,269,443 26,158 2209–7500 (7247–24,606)Others 25,388,311 130,564,541 5143 872,883,419 1,277,775,162 1464Total for Reservoirs 1,713,324,029 28,949,890,852 16,897 3,221,195,376 12,864,541,430 3994*Production data for Louisiana are from the 2002 International Oil Scout Association Yearbook (2006, personal communication), and production data for 2005 for Mississippi are from the Mississippi Oil and Gas Board (2006,personal communication). Reservoir depths for the northern Louisiana salt basin are from the 2006 Quick look handbook onshore Louisiana petroleum producing formations by Goddard (2006, personal communication).Formation depths for the Mississippi Interior salt basin are from well logs in Mancini et al. (1999).2% (96 tcf of gas) was used as the P 90 value. The level ofexpulsion, migration, and trapping efficiency of 2.5%(120 tcf of gas) was used as the P 50 value of the potentialsecondary thermogenic gas distribution.Lewan (2002) reported that a significant part ofthe gas resource in the basins in his interior zone (onshoreinterior salt basins) is a product of the cracking ofentrapped liquid hydrocarbons being converted to gasin deeply buried reservoirs. His conclusion is based onthe premise that maturing source rocks are unlikelyto generate hydrocarbon accumulations with a GORgreater than 1500 scf/bbl. Lewan (2002) reported thatthe mean GOR for the onshore interior salt basins is4280 scf/bbl. Using the GOR criterion of Lewan (2002),the major part of the thermogenic gas in reservoirs inthe Manila subbasin (GOR of 1098 scf/bbl) and theConecuh subbasin (GOR of 1098 scf/bbl with the exclusionof the oil and gas production from the Big EscambiaCreek and Flomaton fields) is a result of thermal crackingof oil to gas in the Smackover source rock (Table 4).The GOR of 1284 scf/bbl in the Conecuh subbasin isbased on the assumption that the gas in the Big EscambiaCreek and Flomaton fields is chiefly a product of thermochemicalreduction of evaporite sulfate in the reservoirsas interpreted by Claypool and Mancini (1989).Thermogenic gas in the reservoirs in the MississippiInterior salt basin (GOR of 3994 scf/bbl) is primarilyfrom the thermal cracking of oil to gas in the Smackoversource rock (Table 3). The thermogenic gas in the NorthLouisiana salt basin (GOR of 16,897 scf/bbl) is probablya result of a combination of thermal cracking of oil to gasin the Smackover source rock and the conversion of oilto gas in deeply buried reservoirs.UNDERDEVELOPED MESOZOIC RESERVOIRS INDEEP GAS PLAYPrevious studies discussed the hydrocarbon reservoir potentialof Upper Jurassic and Lower Cretaceous deeplyburied sandstone and carbonate facies in the northcentraland northeastern Gulf of Mexico area (Puckettet al., 2000; Mancini and Puckett, 2003; Zimmermanand Goddard, 2001; U.S. Geological Survey Gulf CoastRegion Assessment Team, 2006). Puckett et al. (2000)published that Lower Cretaceous carbonate facies ofthe James, Rodessa, Mooringsport, and Andrew (Fredericksburg)formations and sandstone facies of the Hosston,Paluxy, and Dantzler formations represented potentialundiscovered and underdeveloped hydrocarbonMancini et al. 301

302 E&P NoteTable 4. Oil and Gas Production from the Manila and Conecuh Subbasins*Manila SubbasinConecuh SubbasinReservoirOil (bbl) Gas (mcf) GOR (scf/bbl) Reservoir Depth, m (ft) Oil (bbl) Gas (mcf) GOR (scf/bbl) Reservoir Depth, m (ft)Upper Cretaceous – – – –Tuscaloosa 8,916,844 0 0 1615–1646 (5300–5400) 21,542,952 856,043 40 1768–1890 (5800–6200)Lower Cretaceous – – – –Dantzler 111,402 5628 1603 2012–2164 (6600–7100) – – – –Washita-Fredericksburg 1,933,954 81,113 924 2377–2499 (7800–8200) – – – –Paluxy 167,463 243 7094 2530–2652 (8300–8700) – – – –Hosston – – – – 870,845 67,232 77 2713–3048 (8900–10,000)Upper JurassicCotton Valley – – – – 394,618 0 0 2682–3109 (8800–10,200)Haynesville 25,308,648 40,567,820 3414–3810 (11,200–12,500) 4,249,638 944,194 222 3475–4328 (11,400–14,200)Smackover 13,137,966 12,139,263 924 3414–4755 (11,200–15,600) 134,501,360 1,095,832,312 8147 3627–5029 (11,900–16,500)Norphlet 274,345 1,946,083 7094 3688–3749 (12,100–12,300) 19,213,883 257,152,863 13,384 4481–4694 (14,700–15,400)Smackover and Norphlet (FL)472,368,000 609,556,000 1290 4633–4816 (15,200–15,800)Production from Big Escambia77,733,795 1,225,690,112 15,768Creek and Flomaton fieldsSubtotal without production575,407,501 738,718,532 1284 4663–4694 (15,300–15,400)from Big Escambia CreekandFlomatonfieldsTotal for all Reservoirs 49,850,622 54,740,150 1098 653,141,296 1,964,408,644 3008*Production data for 2005 and reservoir depths are from the files of the Alabama and Florida Oil and Gas Boards (2006, personal communication).

Figure 10. Map showing the reference subsea depth of 4572 m (15,000 ft) to the top of the Smackover (SMK), Cotton Valley (CV),Hosston-Sligo (H-S), and Lower Cretaceous (LK) stratigraphic units in the central and eastern Gulf coastal plain (modified fromMancini et al., 2006b, 2007). These stratigraphic reference depth levels assist in a geographic approximation of the occurrence ofdeeply buried reservoir facies with secondary, nonassociated thermogenic gas potential. The 4572-m (15,000-ft) reference level isused because it represents a depth intermediate between the 3658-m (12,000-ft) depth for the strata in the North Louisiana salt basinand the 5630-m (18,470-ft) depth for the strata in the Manila and Conecuh subbasins. These depths are the critical stratigraphic levelsfor the generation of secondary, nonassociated thermogenic gas in the central and eastern Gulf coastal plain.reservoirs in the Mississippi Interior salt basin. Manciniand Puckett (2003) determined that the Norphlet andHosston formations have high potential as deeply buriedgas reservoir facies in the Mississippi Interior salt basin.These formations have produced some 10% of the totalgas volume from this basin (Table 3). Zimmerman andGoddard (2001) concluded that deeper water sandstonefacies of the Hosston Formation had potential as a deepgas reservoir in the southern part of the northern Louisianaarea. In 2002, the U.S. Geological Survey assessedthe hydrocarbon potential of the Cotton Valley andHosston facies in the onshore interior salt basins anddetermined that these sandstones have high gas potentialas undiscovered conventional hydrocarbon reservoirsin these basins (U.S. Geological Survey Gulf CoastRegion Assessment Team, 2006).The Upper Jurassic stratigraphic units account forsome 20% of the current cumulative oil production andsome 38% of the current cumulative gas production inthe Mississippi Interior salt basin and only account forsome 10% of the current cumulative oil and gas productionin the North Louisiana salt basin (Table 3).These Upper Jurassic facies represent potential underdevelopedreservoirs in the North Louisiana salt basin ifthe thermogenic gas resource is preserved in these potentialreservoirs.The number of wells drilled in the North Louisianasalt basin exceeds 100,000 wells (Strategic OnlineNatural Resources Information System, Department ofNatural Resources, State of Louisiana, 2007, personalcommunication), but less than 1% of these wells reacheda total depth of 3658 m (12,000 ft). These drilling statisticsindicate that Upper Jurassic and Lower Cretaceousstrata in this basin have not been adequately tested as totheir hydrocarbon potential.Sequence-stratigraphic study, petroleum systemanalysis, and resource assessment were used in thisstudy to characterize the developing deep gas MesozoicMancini et al. 303

Figure 11. Thermalmaturitymodel calibrationand maturity profilesbased on vitrinite reflectancevalues as measuredfrom core samplesshowing depth to oil andassociated gas generation(R o = 0.55%), thermogenicgas generation(R o = 1.3%), productionlimit of wet-gas generation(R o =2.0%), andcessation of thermogenicdry-gas generation (R o =4.0%) in the source rock.(A) Calibration of modelcomparing calculatedvitrinite reflectance valuesto measured values fromcore samples for theNorth Louisiana salt basin.(B) North Louisiana saltbasin vitrinite reflectanceprofile. (C) MississippiInterior salt basin vitrinitereflectance profile.(D) Manila and Conecuhsubbasins vitrinite reflectanceprofile (modifiedfrom Mancini et al.,2006b, 2007).reservoir play in the central and eastern Gulf coastalplain and to identify areas in the North Louisiana andMississippi Interior salt basins with high potential fordeeply buried gas reservoirs. Areas of the North Louisianasalt basin, especially central Louisiana, have potentialfor deeply buried Mesozoic reservoirs (Figure 10).These potential reservoirs include Upper Jurassic andLower Cretaceous facies of the Smackover, Cotton Valley,Hosston, and Sligo stratigraphic units at depthsgreater than 6096 m (20,000 ft) (Mancini et al., 2006b,2007). Vitrinite reflectance values of 1.8% at a depth of5328 m (17,480 ft) have been reported by Mancini et al.(2006b) for the Cotton Valley Group in this basin. Asshown by this vitrinite reflectance data and by the vitrinite304 E&P Note

eflectance profile in Figure 11B, these Upper Jurassicand Lower Cretaceous strata are projected to be in thethermogenic dry-gas generative and preservation window(R o value range of 2.0–4.0) to 9144 m (30,000 ft)in this basin. Figure 11A illustrates the calibration forthe thermal-maturity modeling performed in this study.Cotton Valley and Hosston potential gas reservoir faciesare expected to be associated with transgressive andregressive systems tract deposits that accumulated inshallow-nearshore and deep-marine sandstone deposits.Porosity is predicted to be a combination of depositionaland diagenetic (fracture) in these sandstone reservoirs.The Smackover and Sligo potential gas reservoir faciesare projected to be associated with carbonate nearshore,marine shelf, ramp, and reef deposits of transgressiveand regressive systems tracts. Porosity is predicted to bediagenetic (vuggy and fracture) in these carbonate reservoirs.The remaining thermogenic in-place gas potentiallyavailable for recovery may total 67–115 tcf in thisbasin.Areas of the Mississippi Interior salt basin, especiallycentral to southern Mississippi (Figure 10), alsohave potential for deeply buried Mesozoic gas reservoirs.The thermogenic gas in these reservoirs is interpretedto be from late thermal cracking of oil to gas inthe source rock and is categorized as secondary, nonassociatedgas based on using a depth limit of greaterthan 5029 m (16,500 ft), which approximates the upperdepth limit for secondary thermogenic gas generationfor this basin. The potential reservoirs includeUpperJurassicandLowerCretaceousfaciesoftheNorphlet,Smackover, Haynesville, Cotton Valley, Hosston,and Sligo stratigraphic units at depths greater than7620 m (25,000 ft) (Mancini et al., 2006b, 2007). Vitrinitereflectance values of 2.0% at a depth of 7309 m(23,981 ft) have been reported by Mancini et al. (2003)for the Smackover Formation in this basin. As shown bythis vitrinite reflectance data and by the vitrinite reflectanceprofile in Figure 11C, these Upper Jurassic andLower Cretaceous strata are projected to be in the thermogenicdry-gas generative and preservation window(R o value range of 2.0–4.0%) to 10,668 m (35,000 ft) inthis basin. Norphlet, Cotton Valley, and Hosston potentialgas reservoir facies are expected to be associated withtransgressive and regressive eolian, fluvial, coastal, andnearshore marine sandstone deposits. Porosity is predictedto be a combination of depositional and diagenetic(fracture) in these sandstone reservoirs. The Smackover,Haynesville, and Sligo potential gas reservoir facies areprojected to be associated with transgressive and regressivecarbonate nearshore, marine shelf, ramp, andreef deposits. Porosity is predicted to be diagenetic (vuggyand fracture) in these carbonate reservoirs. With some13 tcf of gas being produced from the Mississippi Interiorsalt basin (Table 3), the remaining thermogenicin-place gas potentially available for recovery exceeds11 tcf in this basin (Mancini et al., 2006b, 2007).In 1999, the U.S. Geological Survey estimated thecombined total of gas volume for the North Louisianaand Mississippi Interior salt basins as 42.8 tcf(Ahlbrandt, 1999). This total ranked these basins 29 of135 in gas volume of the world’s most gas-prone basins.Although the Smackover Formation is thermallymature for oil generation and expulsion in the Manilaand Conecuh subbasins, this source rock is not thermallymature for secondary, nonassociated gas generationand expulsion in these subbasins (Figures 5F, 11D).Oil is produced from Smackover reservoirs at depths of5630 m (18,470 ft) in fields in Mobile County, Alabama(State Oil and Gas Board of Alabama, 2001). The gasproduced from the Upper Jurassic Smackover and Norphletreservoirs at depths of 4663 m (15,300 ft) in theBig Escambia Creek and Flomaton gas-condensate fields(Table 4) has been interpreted to be chiefly a product ofthermochemical reduction of evaporite sulfate in thereservoirs (Claypool and Mancini, 1989).CONCLUSIONSThe Mesozoic (Upper Jurassic–Lower Cretaceous)deeply buried gas reservoir play in the onshore interiorsalt basins and subbasins in the central and eastern Gulfcoastal plain of the United States has high potential forsignificant gas resources.In the North Louisiana and Mississippi Interiorsalt basins, deeply buried reservoir facies with secondary,nonassociated thermogenic gas potential includeUpper Jurassic to Lower Cretaceous Norphlet, Smackover,Haynesville, Cotton Valley, Hosston, and Sligocontinental, coastal, and shallow- and deep-marine siliciclasticdeposits and carbonate nearshore, marine shelf,ramp, and reef facies. These reservoir facies are locatedin the central part of Louisiana at depths greater than3658 m (12,000 ft) and in the central to southern part ofMississippi at depths greater than 5029 m (16,500 ft).Assuming an efficiency of gas expulsion, migration,and trapping of 2–3%, 96–144 tcf of gas is potentiallyavailable in the North Louisiana salt basin. With some29 tcf of gas being produced from this basin, 67–115 tcfof in-place gas remains. Assuming a gas recovery factorof 65%, 44–75 tcf of gas is potentially recoverable.Mancini et al. 305

APPENDIX 1: KEY PARAMETERS USED INSEDIMENTARY BASIN ANALYSISParameters Gulf of Mexico * Interior Salt Basins **Thermal conductivityof lithosphere 0.0075 cal/cm s jC 0.006 cal/cm s jCThermal conductivity ofsediments0.006 cal/cm s jCChange withlithologyRadioactive heatproduction in uppercrust 0.6 10 12 cal/cm 3 s 0.6 10 12 cal/cm 3 sDensity of sedimentsChange with2.4 g/cm 3 lithologyDensity of crust 2.8 g/cm 3 2.8 g/cm 3Density of upper mantle 3.3 g/cm 3 3.3 g/cm 3Initial thickness ofradioactive layer 8 km N/AInitial crust thickness 40 km 30 kmInitial lithospherethickness 125 km 125 kmInitial temperature ofasthenosphere 1350jC 1333jCExtension factor 3.2 1.25*From Nunn (1984); Nunn and Sassen (1986).**This study.APPENDIX 2: KEY PARAMETERS USED IN GASRESOURCE ASSESSMENTParametersNorth Louisiana Salt BasinArea of basin for hydrocarbon(HC) generation 6.71 10 10 m 2Average thickness of Smackoversource rocks396 mAverage measured TOC* 0.58%Average calculated original TOC** 1.02%Average measured HI for matureSmackover source rocks y39 mg HC/g TOCAveraged determined original HI yy481 mg HC/g TOC*Average measured TOC of 0.58% for mature Smackover source rocks with a thermalmaturitylevel of 0.55% R o or greater in the North Louisiana salt basin is calculatedfrom the TOC values (range of 0.06 – 8.42%) as reported by Mancini et al. (2006b) forthis basin.**Average original TOC was determined by multiplying the measured TOC values by a factor of1.0 – 1.9 (average value of 1.5) depending on the level of thermal maturation (R o percent)of the source rock after Daly and Edman (1987) and Li (2006). Original TOC value of 1.0%was used in the gas resource assessment for the North Louisiana salt basin.y Average measured HI of 39 mg HC/g TOC for mature Smackover source rocks with a T maxtemperature of 437jC or greater in the North Louisiana salt basin is calculated from theHI values (range of 14–94 mg HC/g TOC) as reported by Mancini et al. (2006b) for thisbasin.yy HI range of 306– 656 mg HC/g TOC for immature Smackover source rocks (T max of 422–424jC) as reported by Sassen and Moore (1988) is used to determine the averageoriginal HI.REFERENCES CITEDAdams, R. 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