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.