PETROLEUM SYSTEMS IN THE JIANGLING-DANGYANG AREA, JIANGHAN BASIN, CHINA

Source-related geochemical data define at least four petroleum systems in the Jiangling-Dangyang area of the Jianghan Basin. Eocene-Paleocen e anoxic evaporitic lacustrine source rocks generated most of the crud e oils in the area. Eocene Qianjiang rock from the Sha 13 well (1322 m ) contains fast-reacting, sulfur-rich Type IS organic matter, and its extract is most similar to the Sha 13 oil sand bitumen (Qianjiang rese rvoir). Lower Eocene-Paleocene Xingouzhui rocks from the Xin 73 well ( 842 and 862 m) contain slow-reacting low-sulfur Type I organic matter, and their extracts are most similar to the Sha 26 oil sand bitumen (E ocene Jinsha reservoir) and the Ling 2, Sha 24, and Tuo 3 oils (Xingou zhui reservoirs). Two unidentified Middle Triassic or older marine car bonate-evaporite source rocks or different facies of the same source r ock generated the Daxiakou oil (Triassic Jialingjiang Formation outcro p, Xingshan County) and the moderately biodegraded Tianwan seep oil (P ermian Changxing outcrop, Chengxi County), respectively. One or more u nidentified marine source rocks, which could include the Lower Permian Qixia or the Upper Sinian Doushantuo Formations, generated the Miaosh i and Yanmenkuo seep oils (Permian Qixia outcrops). The Jingshan seep oil (Ordovician Baota outcrop) probably is related to these oils, but could represent another petroleum system. Different kinetics for hydro carbon generation among Eocene Qianjiang and Lower Eocene-Paleocene Xi ngouzhui Formation source rocks and chemical differences among the rel ated oils are caused by organic facies variations. High salinity and l ow Eh enhanced the preservation of oil-prone organic matter in these l acustrine settings and facilitated incorporation of sulfur into the or ganic matter. Anoxia and the unusual presence of abundant sulfate as g ypsum resulted in the microbial reduction of sulfate to sulfide and in corporation of this sulfur into the kerogen. For example, biomarkers s how that source rock in the Sha 13 well (1322 m) was deposited under m ore saline, lower Eh conditions than that in the Ling 80 well (1808 m) , although both are from the Qianjiang Formation. The Sha 13 rock samp le is more organic-rich (6.62 vs. 1.27 wt.% TOC) and has a higher hydr ogen index (794 vs. 501 mg HC/g TOC) and faster reaction kinetics than the Ling 80 sample. Kerogen from the Sha 13 sample is Type IS because it has a high hydrogen index and an atomic S/C ratio (0.074) in the r ange of sulfur-rich, fast-reacting kerogens of the Monterey Formation (S/C > 0.040). Organic-rich Lower Jurassic coaly rocks from outcrops a t Daxiakou contain immature to mature gas-prone organic matter that is not related to any oils in the study. Several organic-rich Upper Sini an to Permian samples could have been source rocks in the past, but ar e now highly mature based on high T-max (464-540 degrees C) and estima ted vitrinite reflectance (R(o)) values. Mass balance calculations wer e used to estimate the original TOC (TOC degrees) in these samples pri or to maturation. These samples could not be correlated with the oils using biomarkers because of high maturities and low extract yields. Ho wever, stable carbon isotope type-curves suggest that the Miaoshi, Yan menkuo (Permian Qixia Formation) and the Jingshan (Ordovician Baota Fo rmation) seep oils originated from source rocks in the Lower Permian Q ixia (3.00 wt.% TOC degrees) or Upper Sinian Doushantuo Formations (5. 96 wt.% TOC degrees). Lack of triaromatic dinosteroids in the Miaoshi and Yanmenkuo seep oils supports, but does not prove a Permian source rock. Very negative stable carbon isotope ratios for kerogens from the Lower Cambrian Shuijintuo Formation (-33.5 to -33.6 parts per thousan d; 8.85-16.64 wt.% TOCO) show that they are not related to any of the analyzed oils. Copyright (C) 1996 Elsevier Science Ltd