Influence of High-Variable Geothermal Background on Hydrocarbon Generation and Expulsion: A Case Study in Baiyun Sag of Pearl River Mouth Basin, Deepwater Northern South China Sea
Due to the geologic events occurred at 23.03 Ma (the boundary of Oligocene/Miocene), post-rift subsidence and sedimentation anomalies in the Baiyun sag (BYS) were intensely influenced by thermal subsidence, which led to a significant increase both of the water depth and terrestrial heat flow, resulted in the present deepwater and high geothermal background. Three sets of source rock were demonstrated in the BYS, including the middle Eocene middle-deep lacustrine source rock (MLSR), the early Oligocene inshore shallow lacustrine source rock (ISLSR) and the late Oligocene transitional source rock (TSR). Contribution of these source rocks has been proven by geochemical research; nevertheless, the majority of the hydrocarbon (especially crude oil) discovered share similar organic geochemical characteristics with the ISLSR. The histories of thermal maturity, hydrocarbon generation and expulsion were modelled. The results showed that all the three sets of source rock have reached or exceeded the hydrocarbon generation thresholds, especially the MLSR which has been in over maturation since late Oligocene. On the account of the large scale, high quality and early maturation, the MLSR was considered the most promising source rock, far more superior than the ISLSR and the TSR. However, the subsection statistics by geological period indicated that more than nearly 70% of the hydrocarbon from the MLSR was generated and expelled before Miocene (23.03Ma), while it was merely less than about 10% for the ISLSR and the TSR. These results suggest that the influence of the geothermal background in the BYS are mainly shown in following aspects: 1) Early maturation caused by the high geothermal background led to the prematurely hydrocarbon generating and expelling process of the MLSR, which couldn't fairly match the development period of the reservoir-cap assemblage in the deepwater sedimentary system (early-middle Miocene), ended up with a limited contribution to the oil and gas accumulation; 2) The increase of paleotemperature after 23.03 Ma expedited the maturation of the ISLSR; 3) High geothermal background decreased the depth of thresholds and made it possible for the source rock relatively shallow (like the TSR) to generate hydrocarbon. In this case, it is the maturation rather than the quality or scale of the source rock that should be focused on in priority order when the source rock under high and variable geothermal background were evaluated.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017