Evaluation of High Quality Source Rocks in Saline Environment of Continental Rift Basin: A Case Study in Laizhouwan Depression, Offshore Bohai Bay Basin, China
Bohai Bay Basin is a petroliferous basin located on the east coast of China. Generally, three organic-rich layers (Es3, Es1 and Ed members) have been proved to be the main sources of oil accumulation. The fourth member of the Shahejie Formation (E2s4) was previously paid little attention as it is featured with deep burial depths of more than 3500m (11483ft) and few source rocks were discovered. However, in our recent investigation, regionally extensive evaporites were discovered in E2s4 Formation of Laizhouwan depression during the Early Eocene when the Bohai Bay Basin was developing as a rift basin. The E2s4 strata is featured with gypsum, halite, mudstone and carbonates deposits. Influence by Tan-Lu fault activities, a salt arch structure was formed. Based on Rock-Eval pyrolysis, total organic carbon, gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS), isotope data of the saturate and aromatic fractions of source rock, and rock trace element data of 150 samples, geochemical characterization of source rocks in saline environment of Offshore Bohai bay basin were analyzed for the first time. TOC contents between 0.28 and 5.09% and S1+S2 values range from 0.17 to 36.24 mg HC/g rock, source rock display strong heterogeneity. Hydrogen indices for E2s4 samples range from 25.02 to 768.77 mg/g TOC, revealing different types. Rock-Eval Tmax values range from 424 to 443°C, indicating that samples mixed with immature and mature for hydrocarbon generation. Low vitrinite reflectance, obvious C35 homohopane preference, low pristane/phytane ratios and high gammacerane indices (0.21-0.78), indicate evaporate or high-salinity environments. Crude oil from Laizhouwan depression has high sulfur contents (higher than 1%, maximum of 2.92%), while other depressions have low sulfur contents. The thermal maturity of crude oil evaluated by analysis of the biomarkers, crude oil from E2s4 displays C29ββ(ββ+αα) sterane ratios between 0.36 and 0.47 which has not reached the endpoint values. The source rock evaluation in saline environment of continental rift basin can be applied for resource assessment and petroleum exploration. We finally analyzed the origin of source rocks in saline environment. Theory of transgression, deep brine and closed evaporation were proposed, of which closed evaporation theory is the most probable origin, we find evaporation is greater than the injection in the enclosed environment and deposits overlying evaporites are often of high quality.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017