Saudi Sandstone Correlations

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The appearance of deltaic/shoreface sediments reflects a change to finer grained, better sorted, more sandy facies in wells at Dilam, Hawtah, Nuayyim and Abu Jifan. Overall, the direction of transport in the basin is towards the east. Marine environment tidal and wavegenerated currents dominate the upper part of this unit. Tidal bundles and bidirectional dips (which indicate wave motion) become more abundant. Dip magnitude and dip azimuth variation are common in the fully marine Upper Ordovician Unit 3. Sediment samples from Haradh mark a shallow eastern shelf of the basin. The basin morphology for Unit 4 (figure 2.17c) also reflects the influence of the Central Arabian Arch. The directional features in the northwest wells - at Dilam and Raghib - trend more to the south than in the basinward wells of Hawtah, Nuayyim, and Hilwah. Wells located in the centre of the Unit 4 basin, at Abu Jifan, Haradh and Khursaniyah, reflect the fact that the axial trough of the basin was dipping to the north. Eastern wells of the Unit 4 basin do not indicate the presence of an eastern shelf. Long shore drift sedimentary dips are more common (in wells at Dilam and Raghib) underlining the marine nature of this tidal deposit. The Central Arabian Arch also played a significant role in the regional basin configuration during the deposition of Unit 5 (figure 2.17d). Palaeocurrent features in the northwestern wells (at Dilam and Raghib) are directed predominantly to the south, in contrast to the more basinward wells located at Hawtah, Nuayyim and Hilwah. The end of the Ordovician was marked by a global glaciation, centred on North Africa. We might expect that the facies and basin configuration encountered in the Upper Ordovician would be very different from those in the Cambrian or Lower Ordovician. In the absence of high-quality deep seismic data, (quality being limited by the thick Khuff Formation) we must understand basin morphology and facies distribution in order to predict the geographical and stratigraphic position of reservoirs. Number 16, 1996. Upper Ordovician thickness Unit 4 Basement DILM ULYH HLWH V V 200 300 400 HWTH ABJF RGHB NYM KRSN ANDR SDGM UTMN UTMN UTMN UTMN HRDH MKSR 400 Fig. 2.17c: The abundance of long shore drift sedimentary dips in Unit 4 (c) indicates a marine environment. Upper Ordovician thickness Unit 5 ABJF Basement DILM ULYH HLWH V RGHB 0 200 V 300 HWTH NYM KRSN ANDR SDGM UTMN UTMN UTMN UTMN HRDH HRDH MKSR HRDH 300 200 0 300 200 0 Isopach contours in feet 0 50 100 km Isopach contours in feet 0 50 100 Fig. 2.17d: Unit 5 is marked by the influence of the Central Arabian Arch, dividing the shallow marine environment into two distinct sub-basins. km 39
Tectonic ups and downs During the Early Ordovician, compressional forces caused a period of uplift in central Arabia. The Central Arabian bulge formed a topographic high which was probably subjected to faulting during the Middle Ordovician. The valleys filled with Middle and Upper Ordovician sediments - including Units 1 to 5 which, although younger than the rocks in the horsts, occur at similar depths. This process, coupled with the lithostratigraphic similarity of the older and younger units and a lack of fossil material for age identification, made the initial interpretation difficult. At present, <strong>Saudi</strong> Aramco explorationists are investigating the tectonic movements which led to the initial uplift, associated erosion and subsequent down-faulting of these central sandstones. These movements were followed by a major depositional period lasting approximately 30 M years which led to the creation of the Upper Ordovician Units 1 to 5 and Sarah/Zarqa sandstone units. Each sandstone was deposited in a different environment. The oldest unit is an (as yet undated) aeolian sandstone, produced in a desert-like environment. This is a clean and well sorted sand with well rounded grains and a consistent cross-bedding direction - all characteristic features of aeolian deposits. Above this are the thicker sediments which make up Unit 2 and Unit 3. These were deposited in deltaic or distal alluvial environments which produced fine-grained rocks with low - angle cross-bedded sands and conglomerates. Unit 4, which overlies Unit 3, comprises a tidal/marginal marine sandstone. Unit 4 also contains the distinctive conglomerate layer which was used as a marker in all of the wells. Above the conglomerate horizon, Unit 4 has the texture of a bioturbated marine sand. Unit 4 is overlain by Unit 5. The Sarah/Zarqa Formation, composed of glacially-derived diamictites has locally Fig. 2.18: This new seismic line shows the Qasim/Saq sequence and the Ordovician deposits which lie above the unconformity. Careful analysis of good seismic data can show the physical and structural relationships between units, but age and stratigraphical correlation required geochemical analysis, borehole imagery and fossil evidence. removed Unit 5 by erosion. The diamictites are poorly-sorted, clastic rocks which contain a wide range of particle sizes. This new understanding of basin development has improved the interpretation of other formations in Arabia. In addition, the re-processing of seismic data, guided by FMS images, is helping explorationists to find tectonic deformations in southern and central Arabia. <strong>Saudi</strong> Aramco geologists are currently reinterpreting old data to help redefine the seismic sequence boundaries and determine unconformities. 40 Middle East Well Evaluation Review
Page 2 and 3: Saudi Sandstone Correlations Large
Page 4 and 5: D e v o n i a n S i l O r d o v i c
Page 6 and 7: 1ft Fig. 2.4a: Tidal bundles from a
Page 8 and 9: Fig. 2.7a: This FMS image from a ty
Page 10 and 11: The supporting matrix is poorly sor
Page 12 and 13: S HAWTAH HILWAH RAGHIB DILAM ABJF T
Page 16: (a) (c) Fig. 2.19: The lower Unit 2

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