塔里木盆地麦盖提斜坡西段石炭系生屑灰岩段旋回划分及意义

从露头剖面及钻井岩芯、电性特征入手,结合区域地质背景认为:麦盖提斜坡西段石炭系生屑灰岩段横向展布相对稳定,夹在上下两套泥岩(或膏质泥岩)之间,呈连续沉积过渡,电测曲线为低伽玛值、高电阻率值,形态总体呈箱形。生屑灰岩段在岩性、电性特征上均具明显的成段性,自下而上具有四次旋回性(相当于五级层序)。Ⅰ旋回层主要为灰质云岩夹膏质灰岩、泥质云岩,电阻率、自然伽玛曲线呈漏斗型,与下伏及上覆地层均呈突变过渡;Ⅱ旋回层主要为含泥灰岩、泥晶灰岩,自然伽玛、电阻率曲线均呈鼓型、箱型特征,与上覆地层呈突变过渡,电阻率曲线幅值相对较高;Ⅲ旋回层主要为泥晶灰岩、泥质灰岩、云质灰岩夹生屑灰岩,电阻率、自然伽玛曲线呈低幅度锯齿型,自然伽玛幅度相对较高,电阻率幅值相对较低,与上覆地层均呈渐变过渡;Ⅳ旋回层以白云岩、灰质云岩为主,电阻率及自然伽玛曲线呈复合钟型特征,与上覆地层均呈突变接触。地层精细划分对比有助于更好地认清生屑灰岩段在纵横向的变化规律,进一步突出有利储集区(琼002—群6井)及有利储集层段(Ⅳ旋回层),为下一步的勘探夯实理论基础。

CYCLOSTRATIGRAPHIC DIVISION AND SIGNIFICANCE OF THE CARBONIFEROUS BIOCLAST LIMESTONE MEMBER IN THE WEST MAIGAITI SLOPE,TARIM BASIN

Outcrop, drillcore, electric property, and regional geological data indicate that the Carboniferous bioclast limestone member in the west Maigaiti Slope represents a regionally consistent stratigraphic unit. It lies between two mudstone (or gypsum mudstone) units with gradual transitional boundaries. Its electric logs are characterized by low GR and high resistivity with the shape of box. The Carboniferous bioclast limestone member can be divided into 4 cyclostratigraphic units (equivalent to fifth-order sequences). These sequences are described below in ascending stratigraphic order. Cyclostratigraphic sequence I consists of mostly limy dolomite with interbeds of gypsum limestone, and argillaceous dolomite. Its electric logs (resistivity and GR) are hopper-shaped, with abrupt changes at the boundaries with underlying and overlying formations. Cyclostratigraphic sequence II is characterized by argilliferous limestone and micrite. Its electric logs (relatively high resistivity, GR) are ovaloid-and box-shaped, with abrupt changes to underlying and overlying formations. Cyclostratigraphic sequence III is mostly micrite, shaly limestone, and dolomitic limestone with bioclast limestone interbeds. Its electric logs (low resistivity and high GR) are serrately shaped, with gradual transitions to underlying and overlying formations. Cyclostratigraphic sequence IC is mostly dolomite and limy dolomite. Its electric logs (resistivity and GR) are compound campaniform with abrupt transitions to underlying and overlying formations. Fine cyclostratigraphic subdivision and correlation of the bioclast limestone member contributes to a better understanding of its regional distribution pattern, helps us to better characterize the favorable reservoir member (cyclostratigraphic sequence IV) and favorable reservoir area (Qiong 002-Qun 6), and provides cyclostratigraphc basis for future exploration.