鄂尔多斯盆地马家沟组准层状岩溶角砾岩特征及其成因*

鄂尔多斯盆地奥陶系马家沟组广泛发育岩溶角砾岩,结构特征复杂。近期研究发现,这些角砾岩的组构和分布特征难以能单纯地用经典的风化壳岩溶模式解释,各类角砾岩的成因机理值得进一步探究。文中基于大量岩心及薄片观察,对苏里格气田东区奥陶系马家沟组上组合不同层位角砾岩进行归纳和判识,明确其空间展布和序列结构特征,分析形成机理。研究发现: (1)根据成因与发育环境,苏格里气田东区奥陶系马家沟组上组合角砾岩可划分为5类,分别是近原地解离角砾岩(B1)、近地表堆积角砾岩(B2)、洞穴堆积角砾岩(B3)、洞顶(壁)碎裂角砾岩(B4)和坍塌角砾岩(B5),它们具有纵向多层叠置、横向可对比的“准层状”特征; (2)不同类型角砾岩的纵向配置和演化与周期性的向上变浅沉积序列和暴露溶蚀有关,共归纳为3种类型,即B1主发育型、B2-B3-B4组合发育型、B5主发育型; (3)准层状角砾岩形成于早成岩期,受控于古地貌起伏背景下的高频海平面变化,由暴露溶蚀透镜体横向迁移连片和纵向多旋回叠置而成。研究结果不仅有助于了解鄂尔多斯盆地马家沟组多样化的岩溶角砾岩特征与成因,也为白云岩早成岩期岩溶研究提供了新的素材。

Fabric types of microbialites from the Stage 3 of Cambrian Series 2 in Sugaitebulake section,Tarim Basin

The Xiaoerbulake Formation(Stage 3, Cambrian Series 2)in the Aksu area of the Tarim Basin is mainly composed of microbial dolostone with its reprehensive section at Sugaitebulake,90 km southwest of Aksu city. This formation can be ideal petroleum reservoirs for its good pores which has been extensively studied. Due to serious diagenesis alteration,however, their features have not been fully identified,which causes difficulty in understanding their formative processes and hampers the petroleum exploration. Here,based on detailed observation on outcrops and thin sections,we identified four types of microbialite fabrics,including thrombolitic fabric,laminated fabric,intraclastic fabric and framestone fabric,and established high-precision vertical evolutionary sequence of microbialites. The thrombolitic and the laminated fabrics can be further subdivided. The Members Ⅰ to Ⅲ are mainly composed of thrombolitic dolostone,thrombolitic-laminated dolostone and laminated dolostone. The Member IV consists of microbial thrombolite mounds,whereas the lower Member V is net-thrombolitic dolostone and the upper Member V intraclastic dolostone and Renalcis framestone. This study reveals that the original fabrics have been affected by dolomitization,recrystallization and dissolution,and the alteration degree of the microbialite fabric is more severe in the lower parts of this formation. The intraclastic fabric has the strongest resistance to diagenesis,followed by the laminated fabric,and then the thrombolitic fabric. The Members Ⅰ to Ⅲ represent tidal flat environments;the Member Ⅳ and the lower Member Ⅴ represent deep subtidal environments;the upper Member Ⅴ represents shallower subtidal environments.