塔中志留系下沥青砂岩段风暴沉积的砂体研究

塔中志留系下沥青砂岩段沉积砂体的类型有滨岸砂、浅海砂坝和陆棚砂3种类型，正常条件下沉积的滨岸砂体具低角度交错层理和少量波痕，呈现出由泥岩-粉砂岩-细砂岩-中细砂岩的反韵律变化特征。风暴作用下，滨岸砂体沉积呈正韵律，多由泥砾向上变化为粉、细砂，生物建造较少，具多方向的低角度板状交错层理、丘状层理。正常条件下沉积的陆棚砂体、浅海砂坝砂体岩性较均一，见交错层理，粒级变化不甚明显，多为粉、细砂岩，以反韵律为主。风暴作用下，砂体底部多具撕裂状泥砾，向上呈正韵律粒序演化，并具侵蚀面、槽状交错层理与丘状层理等特征。风暴作用下的滨岸沉积单砂体相对较厚，一般厚6～13m，宽约数十米，浅海砂坝和陆棚砂的单层砂体相对较薄，厚约3～8m，砂体规模较正常条件下沉积的砂体规模增大。此3类砂体在空间展布上极具规律性，垂向上砂体类型由下而上为滨岸砂-浅海砂坝-陆棚砂的交替演化序列；平面上由海洋向陆地方向砂体类型依次为陆棚砂体-浅海砂坝-滨岸砂体。

Origin analysis of sandbodies of the lower bitumen-bearing sandstone member of Silurian in Tazhong area

Three types of sandbodies were developed in the lower bitumen-bearing sandstone member of Silurian in Tazhong area, i.e.shoreline sandbody, nearshore sandridge sandbody and offshore sandbody. The shoreline sandbody deposited in normal conditions is characterized by cross-bedding with a low angle, a few ripple marks, and the lithology cycle is from siltstone and fine-grained sandstone, and then to meadium-fine-grained sandstone from bottom to top. Under the storm background, the shoreline sandbody mostly changes from muddy conglomerates to siltstones and fine-grained sandstones from bottom to top, with a few bioconstructions, while the low-angle cross-bedding with numerous dip directions, the hummocky cross-bedding is developed. The offshore sandbody and nearshore sandridge sandbody deposited under normal conditions, are homogeneous in lithology, with reverse-graded bedding and cross-bedding being developed, grain size not changing obviously. The sediments are mainly siltstone and fine-grained sandstone. In the storm background, the rip-up of muddy gravels occur at the bottom of sandbody and grain size decreases from lower part to the upper part. The erosional surface, cross-bedding and hummocky cross-bedding are developed. The single sandbody of shoreline deposited in the storm background is relatively thick with thickness of 6-13 m, width about several dozens of meters. The single sandbodies of nearshore sandridge and offshore are thin, with thickness of 3- 8 m. Their sandbody scales are larger than that deposited in normal conditions. The distribution regulation of these three sandbodies is obvious in space. Vertically, sandbody types change from the shoreline sandbody to the nearshore sandridge sandbody then to the offshore sandbody alternately from the bottom to top. Horizontally from marine to continent, the evolution is from the offshore sandbody to the nearshore sandridge sandbody then to the shoreline sandbody.