川东地区达万区块主要滑脱层分布预测

Distribution prediction of the major detachment layers in Dawan Block of East Sichuan

四川盆地东部发育中下寒武统膏盐、下志留统龙马溪组泥页岩、上二叠统龙潭组煤系地层和下三叠统嘉陵江组嘉四段—嘉五段膏盐层共4套主要滑脱层,这4套滑脱层分布及其厚度变化对川东地区构造形成及演变发挥了不同作用,其中中下寒武统膏盐和嘉陵江组嘉四段—嘉五段膏盐滑脱层对川东地区高陡褶皱构造带的形成与发育具有重要控制作用。研究区钻遇这4套滑脱层钻井较少,且多数钻井位于构造高部位,对各套滑脱层分布及厚度变化的认识比较局限,早期二维地震资料品质较差,对滑脱层的识别存在不确定性。研究区大面积连片分布的高品质三维地震资料为滑脱层深入研究创造了条件。通过井下滑脱层测井岩性识别与地层划分,在井震标定的基础之上建立各套滑脱层地震响应关系,并对研究区4套滑脱层开展二、三维地震资料联合解释,采用自然伽马参数岩性反演方法对三维地震资料分布区开展嘉陵江组滑脱层岩性精细预测;利用二、三维地震资料联合预测4套滑脱层厚度分布,厚度预测与井下实钻厚度符合率较高,各套滑脱层厚度平面分布具有明显分区分带性,滑脱层厚度平面变化都具有沿NE—SW向呈明显厚薄相间的条带状分布特征,滑脱层厚度异常变化带与高陡褶皱构造带分布关系密切,嘉陵江组膏盐滑脱层厚度减薄条带与高陡褶皱构造发育带相对应,构造带两翼对应厚度异常增厚条带,其他3套滑脱层厚度变化与构造带发育的关系则与嘉陵江组膏盐滑脱层特征相反。

Four sets of the main detachment layers are well-developed in East Sichuan Basin,including gypsum-salt bed of Middle and Lower Cambrian,mud shale of Lower Silurian Longmaxi Formation,coal measures strata of Upper Permian Longtan Formation and the gypsum layers from Member Jia4 to Member Jia5 of Lower Triassic Jialingjiang Formation.The distributions and thickness changes of these four sets of detachment layers have played different roles in the tectonic formation and evolution in East Sichuan,and the first set of gypsum-salt bed and last set of gypsum-salt detachment layer have important effects on controlling the formation and development of the high-steep fold tectonic belt in East Sichuan.There is uncertainty in the identification of the detachment layers owing to poor quality of 2D seismic data.Few wells were drilled and penetrated these four detachment layers in the study area and most wells were arranged in the high points of the structure,resulting in limited understandings of the distribution and thickness variation of each detachment layer.The high-quality 3D seismic data which is wide-spread distributed in a large area of the study area can provide the conditions for the further study on the detachment layers.Through well logging lithology identification and stratigraphic division of these layers,the seismic response relationship of each detachment layer was established on the basis of well-seismic calibration,2D-3D seismic joint interpretation was carried out for the four sets of detachment layers in the study area,and the natural gamma ray inversion method was used to finely predict the lithology of detachment layer in Jialingjiang formation for the 3D seismic data distributed area.The joint prediction of four sets of detachment-layer thickness planar distribution by using 2D and 3D seismic data has much higher coincidence rate with the actual downhole drilled thickness.The thickness variation of each detachment layer is obviously characterized by the strip distribution,and the planar changes of the thickness is featured by thin-thick interbedded strip in NE-SW direction.The thickness abnormally changed belt of the detachment layer has the close relationship with the distribution of high-steep fold belt for the gypsum salt thickness thinning belt in Jialingjiang Formation corresponds to the developed zone of high-steep fold belt,and two wings of the structural belt correspond to the abnormally thickened strip,but the relationship between the thickness change of the other three sets of detachment layers and the developed structural belts are contrary to the characteristics of gypsum-salt detachment layers in Jialingjiang Formation.