The Ordovician Palaeokarst Reservoir Characteristics And Main Control Factors Analysis In Hanilcatam Region

In recent decade, the fast development of carbonate oil and gas exploration is presented, especially crack-cavity reservoirs in Ordovician carbonate of Tarim basin have been one of the important base of increasing reservoirs and production in our country. Carbonate reservoirs are characterized by height complexity, and the heterogeneity of reservoir development and oil/water distribution of fracture-cavity reservoirs in Ordovician carbonate of Tarim basin is nearly being the world records. As the low structural location of Hanilcatam region in Tabei rise, Tarim basin, buried deep in the6500-8000m, has long been a secondary depression of Tabei rise, and considered to be a marine oil and gas hydrocarbon generation depression, which restrict oil and gas exploration in Hanilcatam region.Based on theories and technologies such as carbonate reservoir geology, seismic historic geomorphology and so on, this paper studied palaeogeomorphology, palaeostream and structural evolution characteristic of Ordovician palaeokarst reservoirs in Hanilcatam region, identifying main control factors of palaeokarst reservoirs, which provide geological foundation of palaeokarst reservoirs search and prediction.In sedimentary facies study, based on regional tectonic evolution setting analysis, Hanilcatam region was in the southern slope of Tabei palaeohigh in the Ordovician, and so the sedimentary environment was open platform facies, in coincidence with Tabei rift. In the Yingshan Formation stage, intra-platform shoals were developed. In the Yijianfang Formation, intra-platform shoals as the major were developed, and patch reefs were developed loacally.Drilling results show that Ordovician reservoirs and oil-bearing intervals are concentratedly distributed in the top of Yingshan and Yijianfang Formation. The middle Ordovician Yijianfang Formation are characterized by calcsparite psammitic limestones, oolitic limestones and glutenite limestones. Generally the grain content is more than80%, and intergranular pores are mainly cemented by calcsparites. Yingshan Formation is lacking of pores, while Yijianfang Formation reef bank reservoirs are better developed, intragranular, intergranular and intercrystalline dissolved pores and moldic pores as the major pores. Core porosity of Yijianfang Formation ranges from0.27%to4.72%, average being1.19%. Permeability ranges from0.0017×10-3μm2to320×10-3μm2, average being3.73×10-3μm2. Log interpretation demonstrated that porosity of Yingshan and Yijianfang Formation ranges from0.117%to72.395%, average being1.42%and permeability ranges from 0.001×10-3μm2to25.004×10-3μm2, average being1.56×10-3μm Comparison analysis of porosity and permeability indicate that reservoir physical properties in horizontal differ from each other.Through the research of Ordovician and Yingshan Yijianfang Formation reservoir pore space types, which is divided to pores, caverns and cracks according to forms and sizes. Pores are mainly composed of intragranular, intergranular and intercrystalline dissolved pores and moldic pores, apertures usually being less than2mm. Caverns sizes vary so much, some up to few meters and are partly and completely filled with calcites and argilla. Cracks consist of structure, pressolution, dissolution crack and so on.As Hanilcatam region effected by multiple stages of tectonic movement and karstification, reservoir development evolution rules and genesis became relatively complex. The karstification intensity in Hanilcatam region is mainly controlled by palaeogeomorphology setting, fellowed by palaeostream and fault distribution and lithofacies. Based on actual palaeogeographic environment and fault distribution characteristics in Hanilcatam region, this region is divided to some districts, which have corrections in reservoir genesis. It is necessary and pracitical to investigate main control factors of these districts.Integrating fault, lithofacies, palaeo-stream, karstification types and palaeogeomorphology in Hanilcatam region, based on main control factors of reservoir development, main control factors belts are divided. Block Ha6is divided to4districts, selectively being fault-palaeostream control area, palaeostream control area close to southern of Tumuxiuke Formation wedge out, southern palaeostream-lithofacies control area and fault-lithofacies control area. Reservoir main control factors differ in each area, resulting in the apparent divergence of reservoir types, distribution characteristics, development scale and connectivity.