四川盆地元坝地区上三叠统须家河组三段致密砂岩气甜点类型与分布

Types and distribution of tight sandstone gas sweet spots of the third member of Upper Triassic Xujiahe Formation in Yuanba area,Sichuan Basin

通过岩心观察、薄片鉴定、测井分析等方法,结合其他地球物理技术,对四川盆地元坝地区上三叠统须家河组三段致密砂岩储层特征进行了研究,结合研究区气井产能特征及主控因素分析,归纳了气藏的甜点类型和成因,并对甜点的分布进行了预测。研究结果表明:(1)元坝地区须三段砂岩类型多样,其中中粗粒钙质岩屑砂岩和细砾岩为优质储层;裂缝类型多样,以低角度剪切缝和成岩收缩缝为主。(2)研究区须三段气藏发育中粗粒钙质岩屑砂岩型和细砾岩型等两大类甜点,结合裂缝特征,可进一步细分为六小类;各类甜点的地质特征、测试方式和试采特征均存在差异。(3)甜点的形成主要受沉积、成岩和构造作用控制,中粗粒钙质岩屑砂岩和细砾岩多分布于三角洲平原和前缘亚相界线附近的分流河道;差异重结晶是砾缘缝形成的原因;构造活动产生的断层和地层挠曲控制了裂缝的分布。(4)中粗粒钙质岩屑砂岩和细砾岩均表现为低GR、低CNL、中—高Rd、中等AC值的特征,优质储层的横波阻抗和μρ(μ为剪切模量,ρ为密度)均表现为中—高值,中粗粒钙质岩屑砂岩孔隙型、细砾岩和穿砾缝叠加型甜点呈条带状广泛分布,其他4类甜点零星分布。

Through core observation,thin section identification and logging analysis and other geophysical technologies,the characteristics of tight sandstone reservoirs of the third member of Upper Triassic Xujiahe Formation in western Yuanba area of Sichuan Basin were studied.Combined with gas well productivity characteristics and main controlling factors in the study area,the types and causes of gas reservoir sweet spots were summarized,and the distribution of the sweet spots was predicted.The results show that:(1)There are various sandstones of the third member of Xujiahe Formation,among which medium-coarse-grained calcareous lithic sandstone and fine conglomerate are high-quality reservoirs.There are various types of fractures,mainly low angle shearing cracks and diagenetic contraction joints.(2)There are two types of sweet spots,such as medium-coarse-grained calcareous lithic sandstone type and fine conglomerate type,which can be further subdivided into six types combined with fracture characteristics.There are differences in geological characteristics,test methods and trial mining characteristics of various sweet spots.(3)The formation of sweet spots is mainly controlled by sedimentation,diagenesis and tectonism.The medium-coarse-grained calcareous lithic sandstone and fine conglomerate are mostly distributed in the distributary channel near the boundary between delta plain and front.Differential recrystallization is the reason for the formation of gravel-edge fractures.The distribution of fractures is controlled by faults and stratum flexure caused by tectonic activity.(4)The medium-coarse-grained calcareous lithic sandstone and fine conglomerate are characterized by low GR,low CNL,medium-high Rdand medium AC values.The S-wave impedance of high-quality reservoirs andμρ(μstands for shear modulus,ρstands for density)are medium-high,and the sweet spots of medium-coarse-grained calcareous lithic sandstone,fine conglomerate and superimposed through gravel cracks are widely distributed in strip shape,while the other four types of sweet spots are sporadically distributed.