Petroleum systems in the Jiangling-Dangyang area, Jianghan Basin, China

Abstract

Source-related geochemical data define at least four petroleum systems in the Jiangling-Dangyang area of the Jianghan Basin. Eocene-Paleocene anoxic evaporitic lacustrine source rocks generated most of the crude 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 reservoir). • • 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 (Eocene Jinsha reservoir) and the Ling 2, Sha 24, and Tuo 3 oils (Xingouzhui reservoirs). • • Two unidentified Middle Triassic or older marine carbonate-evaporite source rocks or different facies of the same source rock generated the Daxiakou oil (Triassic Jialingjiang Formation outcrop, Xingshan County) and the moderately biodegraded Tianwan seep oil (Permian Changxing outcrop, Chengxi County), respectively. • • One or more unidentified marine source rocks, which could include the Lower Permian Qixia or the Upper Sinian Doushantuo Formations, generated the Miaoshi 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 hydrocarbon generation among Eocene Qianjiang and Lower Eocene-Paleocene Xingouzhui Formation source rocks and chemical differences among the related oils are caused by organic facies variations. High salinity and low Eh enhanced the preservation of oil-prone organic matter in these lacustrine settings and facilitated incorporation of sulfur into the organic matter. Anoxia and the unusual presence of abundant sulfate as gypsum resulted in the microbial reduction of sulfate to sulfide and incorporation of this sulfur into the kerogen. For example, biomarkers show that source rock in the Sha 13 well (1322 m) was deposited under more saline, lower Eh conditions than that in the Ling 80 well (1808 m), although both are from the Qianjiang Formation The Sha 13 rock sample is more organic-rich (6.62 vs. 1.27 wt.% TOC) and has a higher hydrogen 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 range of sulfur-rich, fast-reacting kerogens of the Monterey Formation (S/C > 0.040). Organic-rich Lower Jurassic coaly rocks from outcrops at Daxiakou contain immature to mature gas-prone organic matter that is not related to any oils in the study. Several organic-rich Upper Sinian to Permian samples could have been source rocks in the past, but are now highly mature based on high T max (464–540°C) and estimated vitrinite reflectance ( R o) values. Mass balance calculations were used to estimate the original TOC (TOC°C) in these samples prior to maturation. These samples could not be correlated with the oils using biomarkers because of high maturities and low extract yields. However, stable carbon isotope type-curves suggest that the Miaoshi, Yanmenkuo (Permian Qixia Formation) and the Jingshan (Ordovician Baota Formation) seep oils originated from source rocks in the Lower Permian Qixia (3.00 wt.% TOC°) or Upper Sinian Doushantuo Formations (5.96 wt.% TOC°). 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‰; 8.85−16.64 wt.% TOC°) show that they are not related to any of the analyzed oils.