Study On Deep Dolomite Reservoir Of Cambrian Ordovician In Shoal Facies In Bachu Area, Tarim Basin

In the previous research has shown a positive prospect of the deep dolomite reservoir which in the Bachu area of Tarim basin. In upper Cambrian- middle-lower Ordovician, composed by limestone with dolomite layer or dolomite, which contained shoal reservoirs has essential research value. Based on the study of drilling core data, physical analysis data, and use the thin section data, logging data, geochemical date can recover the structure of original rock and the occurrence environment of dolomite. Though the characteristics of petrology, logging curves and stratigraphic sequence try to establish an identification model of deep dolomite shoal reservoir in Bachu area.This study was measured by the theoretical base of carbonate sedimentology and petrology.Through the drilling data and the primary research, we think that the porosity and permeability in shoal reservoir are significantly higher than other interval in upper Cambrian- middle-lower Ordovician dolomite reservoir. In shoal reservoir, the character is mainly controlled by fractures.The reservoir space contains dissolution fractures and caves, intergranular pore, intergranular dissolved pore, intercrystalline pore, intercrystalline dissolved pore.Finally, an analysis of Bachu area of Upper Cambrian Lower Ordovician dolostone reservoir controlling factors.The dolomite in upper Cambrian- middle-lower Ordovician of Bachu area can be divided into three categories,(1) residual oolitic dolomite(grain) dolomite;(2) residual image structure dolomite;(3) crystalline dolomite. The original rock of(1) and(2) types dolomite should be grainstone or oolitic limestone which output in high-energy shoal facies. The original rock of crystallization dolomite should be argillaceous limestone or bioclastic limestone. Carbon and oxygen isotopes date shows that the dolomite which in this area is formed in pene-contemporaneous. The diagenetic environment is a basis of seawater environment with fresh water leaching at later period.The high quality reservoirs in study area should be identifying from following aspects:(1)rock have residual oolitic(grain) structure or residual image; or have fine-crystalline dolomite.(2)in well logging curve, have lower nature gamma(GR) with smooth or wavelet transform shape;meanwhile have relatively low resistivity value. The resistivity curve like “box” shape or “bell”shape indicates a high quality reservoir interval. The high value or volatility significantly of GR curve and the resistivity have high value or abnormally low value indicates poor quality reservoir interval.(3) In Bachu uplift, the unconformity interface, three-grade sequence interface,four-grade sequence interface can represent sea level change. This sea level of change leads stratum expose and suffers fresh water leaching which should controlled the quality of reservoir.At the same time has the characteristics of the above three aspects of the interval which represents the potential high-quality dolomite reservoir layer upon layer, this should be taken seriously in the further exploration and development.