Research On The Preservation Conditions Of Qianbei Depression And Its Margin

Qianbei Depression and its margin locate on the southwestern edge of the upper Yangtze plate, with good raw hydrocarbon accumulation geological conditions. While, its hydrocarbon preservation condition was seriously destroyed by a multi-phase of late stage tectonic movements (Caledonian, Hercynian, Indosinian, Yanshanian and Himalayan). And the oil and gas exploration process was restricted. In this paper, firstly, the impacts of the caprock characteristics, tectonic movements, magmatic activity on the hydrocarbon preservation was discussed. Then, the coupling relationship between hydrological geological and geochemical dynamics and hydrocarbon was researched systematically. At the end, a comprehensive evaluation system for hydrocarbon preservation conditions was set up. The evaluation system was used to grading evaluation and selecting the favorable exploration fields in the Qianbei Depression and its margin, which also provides the basis for oil and gas exploration in the future. The paper mainly has the following results:(1)The sealing ability of cap rock in Qiannan-Guizhong depression was evaluated synthetically through both macroscopic and microscopic study. Results show that the Middle-Lower Cambrian gypsum salt rock, Lower Cambrian Niutitang Formation-Jinding Mountain Formation mudstone, Middle-Lower Silurian Longmaxi Formation-Hanjiadian Formation mudstone are regional cap rocks. The Middle-Lower Cambrian gypsum salt rock in the North West of continuous distribution in the study area,the deposition thickness is general 25-100m.It is I type cap rock, and with good sealing capability. The total thickness of Lower Cambrian Niutitang Formation-Jinding Mountain Formation mud cap rock in Zhenxiong-Hezhang region of depression in north Guizhou and Qingzhen-Xiuwen region of the southeast of Guizhou uplift is above 500m. They are II type cap rock, with middle-good sealing capability. The Middle-Lower Silurian Longmaxi Formation-Hanjiadian Formation mud cap rock show increasing trend of southeast to Northwest and their thickness is thinner in the southeastern depression in northern Guizhou and the north of Qianzhong uplift. They are Ⅱ type cap rock, with middle-good sealing capability. Upper Two Triassic, Middle-Lower Three Triassic cap rock major distribute in Sichuan depression of Tarim Basin. The distribution of residual cover only in syncline depression in northern Guizhou, with local sealing ability.(2)Tectonic action for depression in northern Guizhou depression oil and gas preservation conditions showed regional differences from the south to the north: Denudation thickness in southwest Sichuan is small, covered with thick continental deposit, a small amount of fracture development, overall good preservation conditions; Paleozoic Mesozoic and Paleozoic eroded in Qianbei depression, fracture development is dense, Continental marine cap rocks in erosion and fracture under the overall poor sealing ability; A large area of Qianzhong Uplift exposed Cambrian, partially visible Sinian, marine layer has lost the whole sealing ability, faults intensive development, with poor preservation conditions.(3)Based on the study of distribution and temperature of hot spring, the present meteoric water infiltration depth can be estimated. That of Qianbei Depression margin, e.g. northeast of Yunnan Yiliang, Xifeng of Qianzhong uplift, is above 1600m,with high hydrogeology open degree; While the present meteoric water infiltration depth in Qianbei Depression interior is below 1200m, and with low hydrogeology open degree. Combined with drilling data and hydrodynamic analysis of water environment, from the Qianzhong Uplift to Chuannan depression, tectonic activity decreased, showing the centripetal flow zone-the flow discharge area-centrifugal flow area change trend, the degree of mineralization increases, preservation conditions of oil and gas gets well. The longitudinal analysis, the shallow strata of Qianbei depression with low hydrogeology open degree, deep formation is the major exploration target layer.(4)The comprehensive study of cap rock distribution, tectonism, magmatism, hydrological and geological and geochemical dynamics characteristics shows that: Chuannan depression is the favorable area of oil and gas preservation, but Qianbei depression has generally preservation conditions. Low latent structure, deep area of synclinorium and the southern Sichuan basin margin with relatively better preservation conditions in Qianbei depression are favorable areas for oil and gas exploration in the study area of conventional depression, the lower Paleozoic is the main exploration for conventional natural gas layer.