--> Paleo-Environment Reconstruction and Source Rock Prediction in Brink Bags: A Case Study in Eastern China

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Paleo-Environment Reconstruction and Source Rock Prediction in Brink Bags: A Case Study in Eastern China

Abstract

Hydrocarbon generation mechanism has been regarded as problematic in brink sags because of the small lacustrine area and shallow buried depth in Bohai Sea Area. Recently, some advances in lithosphere thinning brings perfect evidence that the extension and its related lama and fault activities caused the geothermal gradient in Eastern China is far more higher(average 39.5℃/km) than any other area in Bohai Bay Basin(average 28.2℃/km)(Yongan Xue,2018), and plenty of hydrocarbon geochemistry analysis data from drilling well show that the hydrocarbon expulsion threshold is more than 1000m shallower than its adjacent normal basin which makes the hydrocarbon generation possible in brink sags. The source rock quality and its resource potential are the most crucial factors that constrain the prospecting progress in brink sags. So we carried on a composite research to reconstruct the relationship between TOC(total organic content) and paleo-environment of source rock in Qinnan sag based on petrological, paleontology, isotope and element geochemical analyses in Es and Ed Formation in Paleogene and predict the source rock distribution in sparse well zone. The research paleo-water depth based on the sensitive element ratio(Th/U, (Al+Fe)/(Ca+Mg)) exhibit that water turns shallower, and paleo-climate based on the sensitive element ratio (Mg/Sr,Sr/Cu) changed from semi-humid with occasional semi-arid to humid. The paleo-salinity(Sr, Sr/Ba,Ca/Na) from semi-brackish water to fresh water, reduction condition(V/Cr, Ni/Co, Pr/Ph) weakened and the paleo productivity (Al,Ba)declines from Es to Ed. δ13C and δ18O values can be used in climate eliminating the influence of diagenesis in arid climate. The average temperature calculated from δ13C and δ18O values is about 18.8-20.1℃(Zhenhan Wu,2009) which is in agreement with the clay mineral and paleontology results. The correlation between trace element and its ratio with TOC(total organic content) indicates that paleo-salinity and the redox conditions(correlation coefficient 0.83 and 0.74) are the main environmental factors of the development of the lacustrine source rocks and paleo climate as well as paleo-water depth(correlation coefficient 0.60and 0.56) affect it to a certain degree. Trajectory analysis(W. Helland-Hansen,2009) was carried on in Es lacustrine to quantitively calculated paleowater depth and based on relationship between water depth and source rock depth, TOC and well logging information we could display lateral thickness and TOC distribution of source rocks in this brink sag and this method could be put forward to other areas.