Source rock development and oil‑source correlation in eastern China basins
thesisposted on 2022-03-28, 22:12 authored by Huiyuan Xu
In the Dongying Depression, Bohai Bay Basin of eastern China, lacustrine source rocks in the Eocene Shahejie Formation are analogous to those in the Nenjiang Formation in the Songliao Basin and the Green River Formation in Colorado, USA. The rich organic carbon in these types of source rocks (TOC > 10%) has been extensively discussed. However, uncertainty remains as to (1) the depositional environments and the organic matter inputs for adjacent laminae on the millimetre scale, (2) what triggers the change from one laminae to another, and (3) apart from the important algal contribution, what role aerobic and anaerobic bacteria played in the preservation of organic matter. It is necessary to know how subtly different depositional environments (e.g. bottom water anoxia, photic zone euxinia, etc.) and organic source inputs (e.g. bacteria, algae, land plants, etc.) interacted to produce the observed hydrocarbon heterogeneity. Thirty eight low thermal maturity (Ro = 0.3‒0.65%), organic-rich and well laminated bulk shales were subjected to organic geochemical analysis so as to understand the organic matter inputs, the depositional environment, and marine transgressions in the Dongying Depression. The paucity of terrigenous land plant biomarkers, the relationship of aqueous organisms to biomarkers that would suggest terrigenous input, and abundant algal and bacterial biomarkers together support an organic matter input that was primarily controlled by palaeowater conditions. The detection of 24-n-propylcholestanes suggests an additional sporadic marine organic matter input into the Dongying palaeolake. Some shale sample were sliced at the millimetre scale so as to assess hydrocarbon heterogeneity, and thus unveil the precise mechanisms of organic matter accumulation and preservation at the microscale. Significantly, photic zone euxinia was documented on the millimetre scale, hence it was critical for phototrophs to oxidise H2S that was released from bacterial sulphate reduction. The obligate anaerobes significantly degraded and altered the organic matter that was initially provided by phytoplankton in the surface water layers. Micro-changes in palaeowater conditions are shown by varying aryl isoprenoid ratios, relative amounts of isorenieratane, and other markers including the relative abundance of dibenzothiophene, dibenzofuran,methyldibenzothiophene, methyltrimethyltridecylchromans, etc. The sliced samples have been interpreted as embodiments of different levels of euxinic water conditions. These findings reinforce (1) the main aqueous organisms that contributed to hydrocarbons, (2) the association between the open sea and the Dongying palaeolake, (3) the importance of bacterial sulphate reduction in carbon and sulphur cycling, and (4) the anaerobic respiration control on preservation of organic carbon. The controls on hydrocarbon variability at the millimetre scale will enhance our understanding and utilisation of organic geochemistry. This thesis demonstrates comprehensive investigation into organic matter enrichment, the dynamic mechanisms for preserving organic matter, and the applicability of various biomarkers in a lacustrine setting. A combined analysis of biomarkers and compound-specific isotopes was used to discriminate the relative hydrocarbon contribution of mudstones and associated coals to petroleum in the Xihu Depression, East China Sea Basin. Abundant diterpanes (e.g.,isopimarane, phyllocladane) are present in all the samples. For samples dominated by terrigenous organic matter, long chain n-alkanes with a large proportion of gymnosperm inputare approximately 2-3‰ enriched in 13C relative to those with more angiosperm input, regardless of lithology. This provides a way of differentiating specific source contributions of coals and associated mudstones to petroleum.