Shiyou shiyan dizhi (Sep 2022)
A comparison of geochemical features of source rocks of Eocene Wenchang Formation in the deep and shallow water zones of Pearl River Mouth Basin, SE China
Abstract
The Pearl River Mouth Basin (PRMB) is one of the important offshore targets for oil and gas exploration in China and has great potential of oil and gas resources. The Eocene Wenchang Formation is one of the potential source rocks in the PRMB, but its geochemical characteristics are insufficiently studied, particularly for the Wenchang Formation source rocks in deep-water area of the PRMB. Due to the deep burial condition, core samples can rarely be obtained, and most samples are drilling cuttings which are easily to be contaminated by drilling mud, which makes it difficult to directly access the geochemical characteristics of Wenchang Formation source rocks. This paper aims to compare the geochemistry of the Wenchang Formation source rocks in the deep-water and shallow-water areas of the PRMB with an improved catalytic hydropyrolysis method to release the bound hydrocarbons from kerogen macromolecules. Based on the biomarkers and carbon isotopic compositions of individual n-alkanes released, the geochemical characteristics of Wenchang Formation source rocks in deep-water (mainly from Zhu Ⅱ Depression) and shallow water areas (mainly from Zhu Ⅰ Depression) from PRMB are compared. Results indicate that the source rocks of Wenchang Formation in PRMB can be classified into three types including semi-deep to deep lacustrine (WC-Ⅰ), shallow lacustrine (WC-Ⅱ), and semi-deep to deep lacustrine with algal blooming (WC-Ⅲ). The WC-Ⅰ are characterized by low C30 4-methylsteranes (4-Me/C29 < 0.2) and bicadinanes contents, and relatively light carbon isotopic composition of individual n-alkanes (-33‰ to -31‰). The WC-Ⅱ are characterized by low 4-Me/C29 ratios (average=0.06), low to high T/C30H ratios (average=1.04), and relatively heavier carbon isotope composition of individual n-alkanes (-30‰ to -27‰). The WC-Ⅲ are characterized by high content of C30 4-methylsteranes (4-Me/C29=0.66) and heavy carbon isotope compositions of individual n-alkanes (-25‰ to -23‰). The Wenchang Formation source rocks in deep-water and shallow-water areas have semi-deep to deep lacustrine facies (WC-Ⅰ) and shallow lacustrine facies (WC-Ⅱ). In addition, the semi-deep to deep lacustrine source rocks with algal blooming (WC-Ⅲ) also developed in shallow-water areas, but no such type of source rocks were found in deep-water area. This study compared the geochemical characteristics of bound hydrocarbons in source rocks of Wenchang Formation in different regions of PRMB, and found that the semi-deep to deep lacustrine source rocks of Wenchang Formation in shallow-water area can be divided into two types. It provides a scientific basis for a better understanding of the geochemical features of source rocks in different areas of Wenchang Formation in PRMB, and is helpful for oil-source correlations in this area.
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