Genesis of Carbonate Cement and Its Effect on Trap in Donghe Sandstone, Halahatang-Hadexun Area in Tarim Basin
Authors: Chen Xiuyan , Luo Ping , Jia Jinhua , Ma Debo 
Published: March 2015

Abstract
Donghe sandstone of Devonian in Halahatang-Hadexun area is typical clastic reservoir of wave-dominated coast in China. It is buried now more than 5500m.The physical property of Donghe sandstone is well, while the heterogeneity is great for the carbonate cement. Research on genesis of carbonate cement has great significance to reservoir prediction. Based on core observation, this paper indicates there are four lithologic associations of carbonate cement sandstone: biological burrow type(?), bedding - laminae type(?), thin calcareous sandstone intercalated in thick sandstone (?) and calcareous concretion(?), and different genesis of carbonate cement in different lithologic associations. Slice and cathodoluminescence analyses show the carbonate cements include more calcite, some ferrocalcite and ankerite, and few dolomite. The carbonate cement usually deposits after quartz overgrowth, and fills in intergranular dissolved pore, feldspar and lithologic fragments dissolved pore, or occurs metasomatic processes of detrital grain. The results of carbon and oxygen isotope show d13CPDB is -2.8~-7.7‰,average -4.7‰;d18OPDB is -11.8~-16.6‰,average -14.7‰. With comprehensive analysis, the genesis of carbonate cement in Donghe sandstone has four types: organic acid decarboxylation, erosion and solution genesis of Ordovician carbonate, biogenesis and exposed sedimentogenesis.
According to logging analysis, there is a significant negative correlation between the content of carbonate cement and porosity and permeability of reservoir. When the content of carbonate cement is more than 10%, the sandstone cements tight with poor physical property. The dense layers of carbonate cement distribute along unconformity, slope area or structural high part, which forms safeguard as the lateral sealing layers, local floor and cap rocks of hydrocarbon trap. This paper changes the opinion of people that Donghe sandstone as beach sand is not a strongly heterogeneous reservoir and the carbonate cement has no benefit to hydrocarbon trap.

Introduction
Carbonate cement is a kind of authigenic diagenetic minerals, it distributes widely in clastic rock, and it is one factor influencing on reservoir physical property(Kantorowicz,1987;Boles,1998).Many previous studies consider carbonate cement have a preference for sandstones that had relatively high initial porosities and permeabilities. Thus, the flux of formation water and probably long-distance transportation of diagenetic components were more important influences on reservoir quality of sandstones than was the local availability of components(Keith et al.,1991), and the effect of carbonate cement diagenetic phases on permeability was distinct from their effect on porosity(Marco et al., 1991,1993;Sylvia et. al,2000). The carbonate cement mainly occurs within the highest permeability facies in the sandstone body, which results in carbonate cements in sandstones commonly occur as irregularly distributed concretions, which makes heterogeneity of it (Kitty,2001;Shirley et al.,2002). So, research on genesis of carbonate cement has great importance in reservoir prediction and petroleum development. This paper takes Halahatang-Hadexun area, Tarim, as an example, analyzes lithologic associations of carbonate cement sandstone;origin of carbonate cement and genetic characteristics;influences on reservoir property and hydrocarbon trap, and provides new idea to predict beneficial reservoir and find lithologic stratigraphy trap.