Quantitative Evaluation For Sealing Of Caprock And The Controlling Effect In The Central Sichuan Basin

In order to study oil and gas preservation conditions in the central Sichuan Basin, based on making full use of geological, seismic, logging data, development and distribution characteristic of caprock in different layers is studied first. Three sets of caprocks upper Triassic, upper Permian and lower Cambrian caprock. The three mudstone caprocks are determined. They develop all around the study area, which has the characteristic large thickness and wide distribution and is little faulted by fracture. The thickness of Triassic caprock is large, maximum thickness of up to 275 m, in addition to the erosion area, the west high value zone gradually reduces to the East, mud land ratio also has the same trend; the upper Permian caprock distribution in the region, the middle part of Sichuan has a thickness of high value zone 200 m, decreasing thickness of northeast direction, mud-land trend of the southwest is high and the northeast is low. Lower Cambrian caprock is the thickness of the three cover layers, the maximum thickness is up to 450 m. Apart from the west part of the erosion zone, the overall thickness is larger, the thickness of the East is reduced, and the mud to the west is higher, and the east gradually decreases.Study on the characteristics of the three sets of micro sealing characteristics of mudstone caprock analysis from the closed. Based on experimental analysis, three sets of cover layer row displacement pressure characteristics and distribution were studied. The Triassic caprock displacement pressure of the overall performes east high, west low, with a maximum value of about 4.5, is the less pressure caprock of the three sets of caprocks; upper Permian mudstone caprock displacement pressure value higher, which in Weiyuan, Chongqing and Luzhou, Nanjiang have four high value areas, the biggest displacement pressure value can reach more than 40MPa; Lower Cambrian Qiongzhu Temple mudstone caprock displacement pressure from the western high value area decreases to the East, distribution interval between 17.5-60 MPa roof.Gas reservoir residual pressure characteristics are as follows: the Xusi gas reservoirs under the Xuwu caprock in Sichuan area shall be a high pressure gas reservoir, the pressure coefficient distributes on the whole southwest low northeast; the gas reservoir under the Upper Permian mudstone belongs to high pressure gas reservoir, pressure coefficient is over high on the whole data distribution range between 1-1.8; the gas reservoir under the Qiongzhu Temple mudstone is the pressure gas reservoir, the pressure coefficient distributes between 1.1-1.2.The evaluation parameters of the sealing ability of the main cover layer were selected, and the method of grid was used to evaluate the sealing gas capability of the three sets of caprock in the Middle Sichuan area.The result of comprehensive and quantitative evaluation of caprock sealing ability of the Upper Triassic caprock in the Xujiahe group presents the west is high and the east is low, lower Xujiahe oil and gas reservoirs are mainly distributed in the region that the comprehensive evaluation value is less than 0.25; the results of comprehensive and quantitative evaluation of the Permian mudstone caprock sealing ability is higher in west and to the east gradually reduced, the lower Permian oil and gas mainly distributes in the comprehensive evaluation value that is less than-0.01; the result of comprehensive and quantitative evaluation of Cambrian Qiongzhusi formation mudstone caprock sealing ability is less than zero, in Guang’an- Hechuan has a low value area, oil and gas mainly distributed in the comprehensive evaluation value less than-0.15. The control effect of three sets of cover on the oil and gas distribution is effective in space.Through the three cap layer of caprock of gas reservoir in time on the control effects of obtained:three sets of regional cap layer formed effective displacement pressure of time earlier in the period of source rock in the bottom rows of large hydrocarbon, caprock to gas reservoir control in time is effective.