下扬子区孤峰组—大隆组露头剖面特征与岩相变化

Outcrop characteristic and lithofacies changes of both Gufeng and Dalong Formations in lower Yangtze Region

中上二叠统孤峰组和大隆组是华南地区重要的烃源岩勘探目的层位。然而,由于两者在时空分布上都存在明显的岩相变化,使得其在区域地层划分及对比中仍存在较大争议,制约了油气资源勘探及潜力评价工作。因此,本文对下扬子区孤峰组及大隆组的6条典型野外剖面进行详细梳理并总结其岩石地层特征,进一步通过区域地质资料对比,分析其垂向及平面的岩相展布及变化,结合总有机碳含量(TOC)等参数来探讨其页岩油气的勘探潜力。研究结果表明:下扬子区孤峰组岩性厚度变化较大,呈厚薄相间的变化特征,其沉积中心大体位于繁昌—芜湖及句容—溧水一带;孤峰组岩性基本上可分为三段,其中中部硅质岩—泥岩段在区域上稳定发育且厚度较大。大隆组在区域上厚度变化不大,但受古地理格局影响,在区域上岩性分异较明显,其硅质岩段主要发育在皖南地区,而在苏北地区则相变为泥页岩。下扬子地区孤峰组和大隆组TOC高,有机质类型好,成熟度总体上处于生气范围内,具有较大的页岩气资源潜力。孤峰组及大隆组中上段TOC最高,是页岩气有利勘探层位。区域上,无为盆地、句容—南陵盆地、常州—宣广盆地及无锡—苏州地区是页岩气有利勘探地区;而宣广—煤山盆地、句容丹阳—黄桥—泰州地区页岩成熟度较低,具有一定的页岩油勘探前景。

The Middle-Upper Permian Gufeng Formation and Dalong Formation are the important target horizons for the source rock exploration in South China.However,due to the obvious lithologic changes in both spatial and temporal distributions,there is still great controversy in the division and comparison of the regional stratigraphy,which restricts the exploration and potential evaluation of oil and gas resources.Thus here we analyzed six typical field outcrops of the Gufeng and Dalong Formations in the Lower Yangtze region and summarized their lithostratigraphic characteristics.By the regional comparison,we further analyzed their vertical and lateral lithofacies distribution.Combined with the geochemical parameters such as total organic carbon(TOC)content,the exploration potential of shale oil and gas is discussed.The results show that the lithological thickness of the Gufeng Formation in the Lower Yangtze region varies greatly,showing the characteristics of alternating thick and thin,and its deposition center is generally located in the areas of Fanchang-Wuhu and Jurong-Lishui.The Gufeng Formation can be basically divided into three members,among which the middle member with interbedded chert and siliceous mudstone is regionally stable and thick.The thickness of the Dalong Formation does not change much regionally,but it is affected by the paleogeographical setting.The lithological differentiation of the Dalong Formation is obvious in the region,and its chert and siliceous mudstone is mainly developed in the southern Anhui area,while in the northern Jiangsu area,it changes to shale.Based on the previous studies,it is suggested that the Gufeng and Dalong Formations of the Lower Yangtze region have high TOC contents,good organic matter types,and their maturity is generally within the gas generation range,indicating that they have great shale gas potential.The vertical distribution characteristics of the TOC in the Gufeng and Dalong Formations indicate that their middle-upper member is a more favorable horizon for shale gas exploration.Regionally,the Wuwei Basin,Jurong-Nanling Basin,Changzhou-Xuanguang Basin and Wuxi-Suzhou area are the relatively favorable areas for shale gas exploration;the Xuanguang-Meishan Basin and the Jurong Danyang-Huangqiao-Taizhou area have shale oil exploration prospects due to their low maturity.