Prestack inversion identification of organic reef gas reservoirs of Permian Changxing Formation in Damaoping area, Sichuan Basin, SW China

Organic reef reservoirs in the platform margin of Kaijiang-Liangping trough in Damaoping area, Sichuan Basin are thin in single layer, fast in lateral variation, and have small P-impedance difference from the surrounding rock, it is difficult to identify and predict the reservoirs and fluid properties by conventional post-stack inversion. Through correlation analysis of core test data and logging P-S wave velocity, this work proposed a formula to calculate the shear wave velocity in different porosity ranges, and solved the issue that some wells in the study area have no S-wave data. AVO forward analysis reveals that formation porosity is the main factor affecting the variation of AVO type, the change of water saturation cannot affect the AVO type, but it has an effect on the change range of AVO. Through cross-plotting analysis of elastic parameters, it is found that fluid factor is a parameter sensitive to gas-bearing property of organic reef reservoir in the study area. By comparing results of post-stack impedance inversion, post-stack high frequency attenuation property, pre-stack simultaneous inversion and AVO anomaly analysis of angle gathers, it is found that the gas-bearing prediction of organic reef reservoirs by using fluid factor derived from simultaneous pre-stack inversion had the highest coincidence rate with actual drilling data. At last, according to the characteristics of fluid factor distribution, the favorable gas-bearing area of the organic reef reservoir in Changxing Formation was predicted, and the organic reef trap at the top of Changxing Formation in Block A of Damaoping area was sorted out as the next exploration target.

Sequence-lithofacies paleogeographic characteristics and petroleum geological significance of Lower Permian Qixia Stage in Sichuan Basin and its adjacent areas,SW China

Through the analysis of logging,field outcrops,cores and geochemical data,and based on the study of the relationships between sea level changes,sequence filling,paleo-geomorphy and lithofacies,the sequence lithofacies paleo-geography and evolution process of the Lower Permian Liangshan-Qixia Formation(Qixia Stage for short)in Sichuan Basin and its surrounding areas are restored.The Qixia Stage can be divided into three third-order sequences,in which SQ0,SQ1 and SQ2 are developed in the depression area,and SQ1 and SQ2 are only developed in other areas.The paleo-geomorphy reflected by the thickness of each sequence indicates that before the deposition of the Qixia Stage in the Early Permian,the areas surrounding the Sichuan Basin are characterized by“four uplifts and four depressions”,namely,four paleo-uplifts/paleo-lands of Kangdian,Hannan,Shennongjia and Xuefeng Mountain,and four depressions of Chengdu-Mianyang,Kangdian front,Jiangkou and Yichang;while the interior of the basin is characterized by“secondary uplifts,secondary depressions and alternating convex-concave”.SQ2 is the main shoal forming period of the Qixia Formation,and the high-energy mound shoal facies mainly developed in the highs of sedimentary paleo-geomorphy and the relative slope break zones.The distribution of dolomitic reservoirs(dolomite,limy dolomite and dolomitic limestone)has a good correlation with the sedimentary geomorphic highs and slope break zones.The favorable mound-shoal and dolomitic reservoirs are distributed around depressions at platform-margin and along highs and around sags in the basin.It is pointed out that the platform-margin area in western Sichuan Basin is still the key area for exploration at present;while areas around Chengdu-Mianyang depression and Guangwang secondary depression inside the platform and areas around sags in central Sichuan-southern Sichuan are favorable exploration areas for dolomitic reservoirs of the Qixia Formation in the next step.