四川盆地二叠系烃源岩及其天然气勘探潜力(二)——烃源岩地球化学特征与天然气资源潜力

四川盆地二叠系发育中二叠统海相碳酸盐岩和上二叠统海陆交互相碎屑岩两套烃源岩,由于其热演化程度高,造成对烃源岩原始生烃潜力及天然气资源潜力认识不清。为此,通过研究该盆地内探井及盆地周缘剖面大量二叠系烃源岩样品的地球化学特征及生烃潜力随成熟度的变化规律,探讨了二叠系烃源岩的原始生烃潜力以及在地质历史时期生排烃量、干酪根与原油裂解生成天然气的资源潜力。研究结果表明:(1)上二叠统龙潭组泥岩和碳质泥岩总有机碳含量高、原始生烃潜力大,是二叠系中最主要的油气烃源岩,龙潭组煤层也是非常重要的气源岩,上二叠统大隆组有机碳含量和生烃潜力也很高,是四川盆地北部重要的油气源岩,中二叠统碳酸盐岩烃源岩总有机碳含量与原始生烃潜力低,是次要的油气源岩;(2)二叠系烃源岩在地质历史时期生成原油3 290×10~8 t,生成天然气420×10~(12) m~3,龙潭组烃源岩对原油和天然气的贡献率分别为80%和85%;(3)二叠系烃源岩形成的古油藏原油资源量为580×10~8 t,原油裂解气资源量为4.45×10~(12) m~3,干酪根直接生成的天然气资源量约为2.10×10~(12) m~3,天然气资源总量可达6.55×10~(12) m~3,其中原油裂解气占70%。结论认为,四川盆地北部和中部—东南部是二叠系的两个生烃中心,也是最有利的二叠系油气成藏和天然气勘探区域,古油藏是最具天然气资源潜力的勘探目标。

The Permian source rocks and their natural gas prospects in the Sichuan Basin (II): The geochemical characteristics of source rocks and the potential of natural gas resources

There are two sets of source rocks in the Permian of the Sichuan Basin, i.e., the Middle Permian marine carbonate rocks and the Upper Permian transitional clastic rocks. Since the Permian source rocks are currently at the over-mature stage, there is no clear understanding of their original hydrocarbon generation potential and natural gas resource potential. In this paper, the geochemical characteristics and the relationship between hydrocarbon generation potential and maturity of abundant Permian source rock samples taken from the exploration wells in the Sichuan Basin and its peripheral sections were investigated. Then, the original hydrocarbon generation potential of Permian source rocks, the hydrocarbon generation and expulsion during geological history and the resource potential of natural gas from kerogen and crude oil cracking were discussed. And the following research results were obtained. First, mudstone and carbonaceous mudstone in the Upper Permian Longtan Fm are characterized by high organic matter abundance and great hydrocarbon generation potential, and they are the most important source rocks in the Permian. Coal in the Longtan Fm is also very important gas source rocks. The Dalong Fm of Upper Permian is also very high in TOC and hydrocarbon generation potential, and it is the important source rock in the northern part of the basin. While the Middle Permian carbonate source rocks are the secondary source rocks with low organic matter abundance and initial hydrocarbon generation potential. Second, the Permian source rocks generated 3 290×108 t crude oil and 420×1012 m3 natural gas during the geological history, in which the contribution of Longtan Fm source rocks to crude oil and natural gas is 80% and 85%, respectively. Third, the crude oil resources of paleo-oil reservoirs derived from the Permian source rocks are as high as 580×108 t, and the total natural gas resources are up to 6.55×1012 m3, including 4.45×1012 m3 (70%) oil-cracking gas and 2.10×1012 m3 natural gas directly generated from kerogen. It is concluded that the northern and central–southeastern parts of the basin are the two hydrocarbon generation centers of the Permian and they are the favorable areas of Permian hydrocarbon accumulation and natural gas exploration. Moreover, the paleo-oil reservoirs in these two areas are the most prospective targets for natural gas exploration in the Sichuan Basin.