Research Of Ordovician Carbonate Matrix And Fractured-Vuggy Reservior Space In Northern Tarim Basin

Northern Tarim Basin experienced multi-period of structural fracture and karst function collectively, forming a porous-crack-carve domained reservoir in the limestone and dolomite. The kind of reservoir was characterized by old strata, deep burial, and extreme heterogeneity etc. With the enlargement of the exploitation scale, the effect was prohibited by the hysteresis in the foundation of reservoir geology due to the complexity of reservoir space. Thus, it was important in both theory and practice for a deep research in the properties of reservoir space. Focused on carbonate reservoir space, this article studies on rock styles, reservoir spaces, physical characteristics, distribution, origins and control factors based on former results and guided by the instruction of Carbonate petrology and reservoir geology, combined with outcrops, cores, well logs, geophysics and other analytical data.The Yingshan Formation of the lower Ordovician in District 6-7 of Tahe Oilfield consists of micrite, grainstone, dolomitic limestone, dolomite and karst carbonate rock, demonstrating a facies of carbonate platform. The karst carbonate rocks involved collapsed deposition breccia, giant granite, sedimentary rock from underground river, and autobreccia-slump breccia. There was a variety of karst cave infills, of which the chemical part could be classified as the following seven types: slump calcification breccia, ancient calcification, coated conglomerate, calcite, sulphur, gypsum and fluorite. The chemical infill occupied the reservoir space but also preserved intergranular pore and partial residual pore, as well as supported the cave to remit the collapse. The discovery of oil-contained ancient calcification with color of black or yellowish-brown and a smell of sulfuretted hydrogen or bitumen for the fresh surface in Liuhuanggou outcrop, Northern Tarim Basin confirmed that the chemical infills did have reservoir ability.The types for reservoir space in study area were mainly karst caves, corrosion spaces, cracks and pores, which accounted for the reservoir types correspondingly. Among that, big karst cave was the most important reservoir type. Besides, this article certified a potential reservoir kind made of crack and pore in the matrix, which was underestimated before as a tight-typed reservoir. The reservoir rock with reservoir space in matrix was dolomitic limestone and was researched systemically and discussed elaborately from the occurrence, micro-pore and crack, reservoir physical properties, distribution and origin through core thin section, cathode luminescence, scanning electron microscope, energy spectrum analysis and inclusion microscopic temperature measurement etc.The study area underwent seawater diagenesis, freshwater diagenesis and burial diagenesis, which happened mainly in early Caledonian, late Caledonian- hercynian and Indosinian-Himalayan. The diagenesis was varied, of which the positive ones were pressure solution for the emergence of stylolite, corrosion for large amount of secondary pore and fracture or rupture for the cracks, which were meaningful for improving the reservoir properties.Distinctive layers for the cave-typed reservoir, which could be divided as three parts according to the distance from reflection T74; superficial karst zone in 0-60 m, permeate zone in 60-150 m and undercurrent zone in >150m. In the plane, controlled by the major dislocation and karst landscape, the cave-typed reservoir distributed at platform, slope and residual mound along northwest major dislocation like a strip. The properties and origin of karst outcrop was researched by cathode ray, package the cervix temperature, macroelement, carbon and oxygen isotopes, rare earth element and trace element. Also, the evolution stage of karst space and the control factors for the reservoir space such as tectonic movements, faults, ancient landforms, palaeodrainage patterns, rock properties, and diagenesis were discussed.