Origination of natural gas and quantitative calculation of mixed source gas ratio in Baiyun Sag

Baiyun Sag in the Pearl River Mouth Basin encountered few exploration wells of the Paleogene source rocks and various types of source rocks, and developed lacustrine, coal measures, and marine source rocks. There have been long-standing controversies among scholars regarding whether the natural gas in this area is crude oil cracking gas or kerogen cracking gas, as well as whether the natural gas originates from the Enping Formation coal bearing source rocks or from the Wenchang Formation lacustrine source rocks. To solve this problem, we planed to take the gas fields in Panyu Low Uplift and the eastern part of Baiyun main depression as examples, selected the Wenchang Formation and Enping Formation hydrocarbon source rocks drilled in the shallow water area in the eastern part of the Pearl River River Mouth Basin, as well as the crude oils of different genetic types found in the shallow water area and Baiyun Sag, carried out a simulation experiment and product analysis of natural gas generation under high temperature and pressure in gold tube, established the identification chart of natural gas genesis, implemented the genesis model of natural gas in Baiyun Sag, and quantitatively calculated the proportion of natural gas mixed sources. Results show that the natural gas in Baiyun Sag is a mixed genesis gas with properties between coal type gas and oil type gas, and all of them are kerogen cracking gas, with no contribution from crude oil cracking gas. Through the calculation of natural gas mixing ratio, we believed that the natural gas in the Panyu low uplift gas reservoir on the north slope of the Baiyun main depression and the gas reservoir in the east of the Baiyun main depression is mainly generated from the Wenchang Formation source rock, with a contribution ratio of 68% to 100%. However, the maximum contribution of kerogen cracking gas generated by the Enping Formation source rock to the natural gas in this area is only 32%.