塔里木盆地北部下白垩统大型湖相砂坝特征及油气勘探意义

Characteristics and hydrocarbon exploration significance of the huge Lower Cretaceous lacustrine sand bar in the northern Tarim Basin

塔里木盆地北部下白垩统舒善河组是岩性油气藏勘探的潜在领域之一。为了明确舒善河组的古地理背景、有利砂体展布和储层特征,揭示岩性油气藏的勘探潜力,依据露头、钻井、测井和地震资料开展了薄层砂岩刻画与地震属性的综合分析。结果表明:(1)库车坳陷在舒善河组沉积期为炎热干旱气候,水体为中等盐度、浅水、氧化型湖盆环境,沉积岩的颜色以灰褐色为主。(2)塔里木盆地北部发育温宿—西秋、新和—齐满、牙哈—提尔根3个古隆起,走向呈近NE—SW向,北翼陡峭,南翼平缓。沉积在古隆起背景之上的舒善河组,早期具有填平补齐、超覆的特征,发育扇三角洲—湖泊体系;中期为最大湖泛期,以湖泊沉积为主;晚期受古隆起高隆带和水下低隆带的控制,在英买力—东河塘地区一带发育湖相砂坝群沉积。(3)砂坝群呈NE—SW向展布,长140~150km,宽50~60km,面积达7 200km2,砂体累计厚度为10~50 m,发育低角度交错层理、沙纹层理、浪成波痕、反粒序等沉积构造,自然伽马测井曲线以漏斗形和齿化箱形为主,地震反射剖面显示砂体呈透镜状、具有叠瓦式侧积的特征。(4)舒善河组的砂坝现今处于北倾斜坡带,南部有4 km宽的泥岩带封堵,在纵、横向上有断裂和不整合面沟通油气源灶,有利于形成砂岩上倾尖灭型圈闭带。塔里木盆地北部下白垩统舒善河组的有利圈闭面积为800km2,砂岩厚度为10~25 m,具有亿吨级储量潜力,是岩性油气藏效益勘探的最有利区。

Lower Cretaceous Shushanhe Formation in the northern Tarim Basin is a potential exploration field of lithologic reservoir.In order to clarify the paleo-geographic background,favorable sandstone distribution and reservoir characteristics of Shushanhe Formation as well as reveal the exploration potentials of lithologic oil and gas reservoirs,thin sandstones are described and seismic attributes are comprehensively analyzed on a basis of outcrop,drilling,logging and geophysical data.The Shushanhe Formation in Kuqa depression is dominantly developed in a hot arid climate and oxidized lake-basin environment with moderate salinity and shallow water,and the sedimentary rocks are mainly in grey brown.Three paleo-uplifts are developed with nearly NE-SW trending in the northern Tarim Basin,i.e.,Wensu-Xiqiu,Xinhe-Qiman and Yaha-Tiergen,which were gentle in the south while steep in the north.The Shushanhe Formation,developed on the paleo-uplift tectonic setting,was characterized by filling and overlying in the early period with the development of fan delta and lake system,maximum lake flooding in the middle period with the development of lacustrine sediments,and developing lacustrine-facies sand bar groups upon high paleo-uplift and underwater low uplift from Yingmaili to Dongtanghe area in the late period.Sand bar groups were distributed in NE-SW trending with lengths of 140-150 km,widths of 50-60 km,an area of 7 200 km2 and accumulated sand thicknesses of 10-50 m,where the low-angle cross-bedding,sand bedding,wave ripples,reverse sequence and other sedimentary structures were developed.The gamma logging curve was dominated by funnel shape and gear box shape,and seismic reflection showed the characteristics of lenticular sand bodies with imbricate lateral accretion.Sand bars in Shushanhe Formation are located on the north steep slope,and the south of Shushanhe Formation is blocked by a4-km wide mudstone zone.The faults and unconformable surfaces are connecting to oil and gas source kitchens in the vertical and horizontal directions,favorable to the formation of up-dip pinch-out sandstone trap.The favorable trap area of the Lower Cretaceous Shushanhe Formation in the northern Tarim Basin is 800 km2 with the sandstone thickness of 10-25 m and the hydrocarbon reserves of hundred million tons,which is the most favorable region for lithologic reservoir efficient exploration.