Different types of Paleogene source rocks and characteristics of pyrolysis gas generation of crude oil in Zhuyi Depression, Pearl River Mouth Basin

HZ26-6 condensate gas field is the first major discovery of condensate gas reservoir in Zhu Ⅰ Depression in the Pearl River Mouth Basin （East）, breaking the traditional situation of exploration around crude oil in Zhuyi depression in recent 40 years, and revealing the huge natural gas exploration potential of oil-rich depression in this area. However, systematic research on the gas generation mechanism and gas generation potential of deep lacustrine source rocks in Wenchang Formation （Fm） and river biogas coal-bearing source rocks in Enping Fm are scarce. Therefore, we conducted a thermal simulation experiment of gold-tube-autoclave closed system to analyze systematically the process of natural gas generation from different types hydrocarbon sources in typical Wenchang Fm and Enping Fm in Zhu Ⅰ depression and the evolution characteristics of carbon isotopes of natural gas, to explore the genesis of natural gas and main source rocks in Huizhou 26-6 condensate field. Results shows that the maximum cumulative hydrocarbon gas yield of lacustrine coal-bearing mudstone of Enping Fm in Zhu Ⅰ depression is 230.8 mL/g.TOC, and the methane gas production rate increases exponentially after 1.1% Ro, being characteristic of continuous gas generation, mainly kerogen pyrolysis gas; The maximum cumulative hydrocarbon gas yield of deep lacustrine mudstone in Wenchang Fm is 428.30 ml/g.TOC. A large amount of gas is produced after the ～1.6% Ro stage, and the high gas yield is actually mainly contributed by the secondary cracking of oil; The maximum yield of crude oil cracked gas is 693.6 mL/g.TOC, which is much higher than that of mudstone in Wenchang Fm and Enping Fm. In addition, differences are obvious in the evolution characteristics in carbon isotopes of natural gas components generated by different types of source rocks and crude oil samples. The carbon isotopic compositions of methane and ethane of the natural gas generated by the partial sapropelic mudstone of Wenchang Fm in Zhu Ⅰ depression in the stage of 1.3%～1.6% R_o are well matched with the natural gas of Huizhou 26 condensate gas reservoir, the main gas source rock of Huizhou 26 condensate gas reservoir. The lacustrine coal-bearing mudstone and raw oil pyrolysis gas of Enping Fm have no obvious contribution to the gas reservoir formation.