Organic geochemistry, distribution and hydrocarbon potential of source rocks in the Paleocene, Lishui Sag, East China Sea Shelf Basin

The Lishui Sag, one of the most promising hydrocarbon exploration areas in the East China Sea Shelf Basin, has had no major breakthroughs after its exploration and development. Therefore, it is crucial to carry out a comprehensive assessment of this basin to illustrate the potential and distribution of source rocks in the Lishui Sag. Based on organic petrographic examinations, Rock-Eval pyrolysis, gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) analysis, the organic geochemical characteristics and paleoenvironment of the source rocks were investigated. The results imply that the kerogen type of the source rocks in the Lishui Sag is dominated by Type III, with a small amount of Type II kerogen. Biomarkers show that the source rocks are characterized by a mixed origin of organic matter in a transitional environment. The lower Mingyuefeng (E(1)m(1)), lower Lingfeng (E(1)l(1)) and Yueguifeng (E(1)y) Formations show higher inputs of aquatic organic matter. The base limits of the total organic carbon (TOC) values of effective source rocks were determined to be 0.9 wt % in the E i m Formation, 1.0 wt % in the E(1)l Formation and 0.7 wt % in the E(1)y Formation. Based on an artificial neural network (ANN) model and the Delta log R method, the distribution of effective source rocks in the Lishui Sag was illustrated. Source rocks in the E(1)m(1) and Lingfeng (E(1)l) Formations can be expected to generate mixed liquid hydrocarbons, while samples from E(1)y Formation are oil prone source rocks with normal oil and light oil potentials. Additionally, the post mature source rocks of the E(1)y Formation may be the origin of natural gas. Upon full consideration of the potential and distribution of source rocks, the North part of the Lingfeng Uplift is the most favorable exploration zone.