H2S formation and enrichment mechanisms in medium to large scale natural gas fields (reservoirs) in the Sichuan Basin

A comprehensive study on the temporal and spatial distribution of H2S accumulations and the geochemical characteristics of sour gas fields (reservoirs) in the Sichuan Basin has been conducted. It has been followed by a series of carefully designed simulation experiments to elucidate an H2S formation mechanism which is able to predict the distribution of H2S. In some reservoirs, for example P2ch(T1f, have sufficient magnesic SO42( and abundant hydrocarbons (palaeo pools) present at sufficiently high temperatures to cause thermochemical sulfate reduction (TSR). This is the major origin of abundant H2S in large to medium scale gas fields. The formation mechanism of highly sour gas fields (reservoirs) is an in-situ reaction under conditions where sufficient source, enough energy supply, suitable reservoir volume and severely reducing environments occur. However, in some areas or strata, such as slope belts or on continental shelf deposits, or formations such as the Jialingjia and Leikoupo Formations, which are dominantly compact carbonatite and have contemporaneously different facies, it is difficult to form abundant H2S. These formations lack a source of sulphur and energy compared to the P2ch(T1f reef flat facies reservoirs. Even though highly sour natural gas migrating from the P2ch(T1f reservoirs is present, H2S is often consumed due to long distance migration and results in hydrocarbon-rich sweet natural gas. This study predicts that the Heichiliang reef flat facies sedimentary zone in northeastern Sichuan to have highly sour natural gas reservoirs. 摘要: ： 通过对四川盆地含硫天然气田（藏）时空分布、地球化学特征及模拟实验结果等的综合研究，揭示H2S形成机理并进行分布预测。P2ch—T1f储集层中足够的含镁SO42-溶液和丰富的烃类（古油藏）在高温条件下发生硫酸盐热化学还原反应（TSR）是四川盆地大中型天然气田中大量H2S形成的主要途径；充足的物源基础、足够的能量供给、适宜的储集空间和苛刻的还原环境下发生的原地反应是高含硫天然气田（藏）形成的主要机制。而以与P2ch—T1f礁滩相储集层同期异相的致密碳酸盐岩为主的斜坡带、陆棚内部以及嘉陵江组、雷口坡组等因缺乏物源、能量等难以形成高丰度H2S，即使有自P2ch—T1f储集层运移而来的高含硫天然气，也因远距离运移H2S被消耗而变为富烃低硫天然气。预测川东北黑池梁礁滩相沉积区有发育高含硫天然气藏的可能性。图4表3参30 Key words: northeastern Sichuan, H2S, TSR, enrichment mechanism, highly sour gas fiel

New knowledge of hydrocarbon generating theory of organic matter in Chinese marine carbonates

The marine carbonates of the Tarim Basin are taken to study the relationship between source rock and oil and gas reservoirs and discuss the possibility and mechanisms of mature and low-TOC (less than 0.5%) marine carbonates being source rock. By studying the matching relationship between source rocks in Tarim Basin and hydrocarbon reservoirs in Tahe oilfield and analyzing the hydrocarbon-generating organism combination and its correlation with geochemical characteristics of oil and gas reservoirs, this study established a method to evaluate hydrocarbon generation substance of marine carbonate source rocks. It is concluded that the crude oil of the Tahe oilfield in the Tarim Basin is not derived from muddy source rocks, but has the obvious characteristics derived from carbonate source rocks. It revealed that the underestimated hydrocarbon-generating substance (organic acid salts) in the highly evolved marine carbonate rocks and high quality hydrocarbon-generating organism are the key to high-evolution carbonate rocks being as source rock. Organic acid salts have high hydrocarbon conversion rate and are mainly cracked into natural gas at high temperature. The development model of the source rocks of the Cambrian-Lower Ordovician carbonate source rocks in the Tarim Basin is dominated by the shelf model. Key words: Tarim Basin, carbonate rocks, hydrocarbon source rock, hydrocarbon-generating organism combination, organic acid salts, hydrocarbon generating theor