25th International Meeting on Organic Geochemistry IMOG 2011

P-125
Organic geochemical and petrographic characterization of
fluvial-lacustrine source rocks and implications for hydrocarbon
source correlation in Cenozoic rift basins, NE China
Maowen Li 1 , Xue Wang 2 , Shuzhi Wang 2 , Julito Reyes 3 , Sneh Achal 3 , Zihui Feng 2 , Wei
Fang 2
1 Sinopec Research Institute of Petroleum Exploration and Production, Beijing, China, 2 PetroChina Daqing
Oilfield Company, Daqing, China, 3 Geological Survey of Canada, Calgary, Canada (corresponding
author:limw@pepris.com)
Bulk and molecular organic geochemical analyses of
source rock and oil samples from the Fangzheng,
Tangyan and Hulin rift basins, NE China indicate the
presence of fluvial and lacustrine-sourced petroleum
systems in this frontier exploration region. Samples of
potential source rocks include the Eocene-Oligocene
coal, oil shale and lacustrine mudstone litholohgies
and represent a variety of depositional environments.
Rock-Eval and total organic carbon data from these
samples reflect generally high quality source rocks,
including both oil- and gas-prone kerogen types, in
the early to peak stages of hydrocarbon generation.
Petroleum source rocks in the study area generally
belong to the fluvial-lacustrine facies association,
characterized by freshwater lacustrine mudstones and
oil shales interbedded with fluvial-deltaic deposits
including coal. Shoreline progradation dominated
basin fill, resulting in the stacking of indistinctly
expressed sedimentary cycles. Transported terrestrial
organic matter contributes to mixed type I-III kerogens
that generate waxy oil. Based on whole-rock organic
petrographic observations, we propose a three-fold
organic facies classification in the study area: (1) oil
shales and laminated mudstones containing oil-prone,
type I kerogens, mainly in the form of Prasinophyte,
filamentous and coccoidal algal mats; (2) silty
shales containing mixed vitrinite, sporinite and
Prasinophyte and filamentous alginite macerals;
and (3) coals and carbonaceous shales that are
enriched in vitrinite, with trace to variable
sporinite, suberinite, cutinite, and resinite.
Results of open-system pyrolysis experiments
reveal hydrocarbon generation kinetics typical of
type I, I-III and III kerogens respectively. Thus,
hydrocarbon generation from type III source
rocks begins at about 0.6%Ro, with almost 75-
80% of liquid hydrocarbon potential having been
realized at ~0.8 %Ro. In contrast, hydrocarbon
generation from type I source will not begin prior
to 0.7 %Ro, with only 20-25% of liquid
hydrocarbon potential realized at 0.8 %Ro.
Bulk geochemical and biomarker data indicate that
organic rich fluvial-lacustrine facies in the Eocene
Xinancun Formation is the most likely source rocks in
these Cenozoic rift basins. Oils and source rocks from
the Tangyan Trough demonstrate geochemical
characteristics consistent with lacustrine
environments and indicate strong evidence for algal
input into fresh to brackish-water source facies
including elevated relative concentrations of C28
steranes. Despite similarities between oils from the
Fangzheng and Hulin troughs, biomarker parameters
suggest higher algal input in facies sourcing
Fangzheng oils compared to Hulin oils. Based on
sterane isomerisation ratios and diamondoid
concentrations in the oil, the thermal maturity levels of
discovered oils tend to increase from Hulin,
Fangzheng to Tangyan trough, consistent with the
change in the dominant source kerogen types.
The hydrocarbon gases in these rift basins occur
either as biogenic gas pools, or as thermogenic gases
associated with the oil pools. The thermogenic gases
are interpreted to have been generated from the
Eocene-Oligocene fluvial-lacustrine source rocks, with
mixed type I and type III sources, and the biogenic
gases are more likely derived from the coarser parts
of the source beds that have a predominance of
terrigenous plant organic matter and are more likely to
experience freshwater infiltration .
Fluvial-lacustrine sediments in these rift basins
generally contain vertically stacked cycles, and are
laterally discontinuous with strong facies contrasts.
Thus, it is not easy to define a single oil window in a
given basin, but it is common to observe large
fluctuations in the properties of reservoired
hydrocarbon fluids, both vertically and laterally.
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