Major controlling factors for the formation of inter-layer of Donghe sandstone of Donghe1 reservoir in Tarim Basin and its fluid source

Through massive cores and thin sections observation, and according to the types and content of interstitial materials, inter-layers of Donghe sandstone in Donghe1 reservoir were classified into three categories: calcareous inter-layer, mud-calcareous inter-layer and mud inter-layer. Calcareous inter-layer, mud-calcareous inter-layer are predominant while mud-calcareous inter-layers are less developed. The formation of inter-layers is primarily controlled by carbonate cementation in the early diagenetic stage. Carbonate cements include calcite, siderite, ferrocalcite and ferrodolomite. Four phases of carbonate cements with different causes and fluid sources were identified according to the different petrographical and cathode luminescence characteristics. The first-phase cements is inter-growth or coarse flaky, iron and magnesium free calcite with low content of Fe and Mn, formed under non buried or shallow buried open oxidizing environment which is connected with seawater. Calcium originates from seawater. The second-and third-phase cements are precipitated between point-line contacted particles. Mn content is increased significantly, formed under semi closed or closed diagenetic environment. Calcium fluid is sourced from the first-phase cements or syn-sedimentary dissolution of carbonate particles. The fourth phase cements were ferro-calcite and ferro-dolomite with replacing quartz and feldspar features. The content of Fe was much higher than that of Mn. It formed under relatively deep buried reducing environment, with relevance to the dissolution of feldspar. ©, 2015, Science Press. All right reserved.