川南地区二叠系沉积环境及其演化特征——以四川古蔺芭蕉村剖面为例

Characteristics of sedimentary environment and evolution of Permian in southern Sichuan Basin: An example from the profile of Gulin Bajiaocun in Sichuan Province

关键词:  沉积环境  碳酸盐岩缓坡  天然气勘探  二叠系  四川盆地

Key words:  sedimentary environment  carbonate slope  gas exploration  Permian  Sichuan Basin

摘要:

以四川古蔺芭蕉村二叠系剖面为研究对象，在野外沉积特征详细观测的基础上，利用岩石薄片鉴定、全岩X衍射分析、元素地球化学与碳、氧、锶同位素分析，对其地层、沉积环境与演化特征进行研究，为四川盆地二叠系沉积特征研究与天然气勘探提供基础资料。研究表明，四川古蔺芭蕉村地区从中二叠世的梁山—栖霞组沉积时期开始海侵，栖霞组由下向上水体先变深又变浅，呈缺氧—贫氧环境；茅口组底部又一次海侵，以继承栖霞组沉积环境为特征，在茅口组一段水体最深，向上逐渐变浅，呈缺氧—贫氧特征；龙潭组沉积水体最浅，氧化环境为主，局部呈还原环境；长兴组底部沉积水体最深，整体呈缺氧—贫氧特征，其碳酸盐岩沉积的海平面相对中二叠统应较浅。宏观上，四川盆地中二叠统栖霞组、茅口组由东向西为碳酸盐岩缓坡逐渐向台地演化。茅口组三段发育大型的溶蚀孔洞和后期溶蚀缝洞，推测该区茅口组有可能形成岩溶缝洞型储层。长兴组的底部发育水体较深的台盆次相沉积，环台洼或台盆可能是礁滩发育的有利区。见大量的沥青质充填，考虑到川南地区二叠系发育富有机质的碳酸盐岩与黑色页岩及煤层，认为该区二叠系具备形成古油藏的可能。

Abstract:

This paper takes the profile of Gulin Bajiaocun in Sichuan Province as the research object to figure out the characteristics of sedimentary environment and evolution of Permian in southern Sichuan Basin. Based on the detailed observation of sedimentary characteristics in the field profile, using the rock slices identification, whole rock X-ray diffraction analysis, element geochemistry, and carbon, oxygen, strontium isotope analysis, characteristics of the profile's formation, sedimentary environment and evolution are studied. It can be figured out that transgression began from the depositional period of the Liangshan Formation to the Qixia Formation, and the water body of the Qixia Formation became deeper and shallower from bottom to top, presenting an anoxic-poor oxygen environment. The bottom of the Maokou Formation was transgressed again, which inherited the sedimentary environment of the Qixia Formation. The water in the first Member of the Maokou Formation was the deepest, and it gradually became shallower upward, presenting the characteristics of anoxic-poor oxygen. The Longtan Formation was with the shallowest sedimentary water, presenting the mainly oxidized environment and partially reduced environment. The bottom of the Changxing Formation was with the deepest sedimentary water, which was characterized by anoxic-poor oxygen. The sedimentary environment of the Permian Qixia and Maokou Formation gradually evolved from carbonate slope to platform from east to west in Sichuan Basin. Large dissolution vugs developed in the third Member of Maokou Formation. It is speculated that karst vuggy reservoirs may be formed in this area. There are platform basin suborder facies deposits of deep water in the bottom of the Changxing Formation of this field profile. The peripheral area around the platform depression or platform basin may be the favorable area for reef and shoal development, which is beneficial to the formation of reservoir. A large amount of asphaltene filling can be seen in the carbonate rocks in this section. Considering the development of organic-rich carbonate rocks, black shale and coal seams in the Permian in southern Sichuan Basin, it is considered that there is a possibility of paleo-oil reservoirs in the region.