Characteristics And Genetic Mechanisms Of The Carbonate Reservoirs Of Middle-Lower Ordovician,Shuntuoguole Area,tarim Basin

Deep to ultra-deep carbonate is one of the most important regions for oil and gas exploration in petroliferous basins on the current.Many oil and gas discoveries and breakthroughs have been made in deep to ultra-deep carbonate reservoirs.The formation and maintenance of carbonate reservoirs under deep and ultra-deep conditions are controlled by various factors such as dolomitization,structurally hydrothermal fluid activity,and so on.The particular geological background of different areas determines the complexity of deep and ultra-deep carbonate reservoirs.Thus,the author selected carbonate of Middle-Lower Ordovician(with depth of about 6000-8000 m currently)in the Shuntuoguole area of the Tarim Basin.Based on the cores of wells,the author carried out research work including detailed core investigation,petrographic analysis,fluid inclusions,isotope analysis,and trace elements to discuss the formation mechanism of different types of carbonate reservoirs.Dolomite reservoirs,silicified carbonate reservoirs and limestone reservoirs are developed in the study area.The dolomite reservoir in the study area is located in the lower Yingshan Formation,but the origin of the dolomite reservoir is still controversial,and which constrains further exploration and deployment.There are few samples of dolomite reservoirs unless the Shunnan 501 well even then a certain amount of cores in different wells.Detailed core investigation,micropetrography,diagenesis event sequence,pore image analysis of casting thin section,computed tomography,backscattered image and trace element Fe/Mn quantification of electron probe,fluid inclusion were carried out to study the characteristics and genesis of dolomite reservoirs.The type of dolomite reservoir is fracture-pores and the main reservoir space is intercrystalline pores and intercrystalline solution pores together with fractures and expansion fractures.There is a positive correlation between pores and fractures on the occurrence.Fluorite and calcite as symbiotic hydrothermal minerals filled in fractures and pores.The dolomites and dolomite's ring edge and calcite coexisted with fluorite nearby fractures and pores show high contents of FeO and MnO.Groups of colorless transparent fluid inclusions in fluorites have 165?175? of homogenization and 15.5?17.5 wt.%NaCl equiv.of salinity.Hydrothermal fluid activities should be resulted in Fe and Mn elements being rich in calcites and dolomites of surrounding rock,and at same time provided the F needed for fluorite crystallization.On the one hand,hydrothermal fluid reconstructs the dolomite to form the reservoir.On the other hand,the hydrothermal minerals as cements represented by the fluorites and calcites filled in fractures and pores.Thus,the generation of dolomite reservoir spaces should be related to tectonic-hydrothermal fluid activity.This understanding of dolomite reservoir in well SN501 is great significant to the establishment of dolomite reservoir formation model and further exploration in this area.An unusual,silicified,carbonate reservoirs which is represented by Well Shunan 4 was found in the upper Yingshan Formation in the study area.According to the degree of silicification,the silicified reservoirs can be divided into a lower section of silicified carbonates,a middle section of limestone,and an upper section of silicified carbonates.The silicified carbonates are mainly composed of quartz and calcite,in which the reservoir space mostly occurs as vugs,inter-crystalline pores of quartz,and partial fractures.Porosity varies widely,ranging from 3-20.5%with strong heterogeneity.The homogenization temperatures of fluid inclusions in quartz and calcite show that the silicification temperatures were 150-190?,with characteristics of high temperature/low salinity and low temperature/high salinity.The 87Sr/86Sr ratios of secondary calcite are 0.709336-0.709732,which are significantly higher than that of concurrent seawater,indicating that the hydrothermal fluid originated from the deep clastic strata or the basement(sialic rock).The ?13C values of the secondary calcite are similar to that of the surrounding limestone,indicating that the carbon in the secondary calcite is derived from the limestone strata,and that the secondary calcite is the product of dissolution and re-precipitation resulting from interaction between the silica-bearing hydrothermal fluids and surrounding limestones.The silicification of silica-bearing hydrothermal fluid was significantly controlled by strike-slip faults.The fluids ascending along the fault zone and branch faults interacted with the surrounding limestone in the Yingshan Formation.As a result,a large amount of quartz and secondary calcite were formed together with various types of secondary pores,resulting in excellent reservoirs.The limestone reservoir in the study area is located in the Yijianfang Formation of the Millde Ordovician,the reservoir space of limestone reservoir is dominated by pores and micro-pores revealed by the cores and thin sections.The characteristics and genesis of pores and micro-pores are described by computed tomography,thin section analysis,Cathodoluminescence analysis,scanning electron microscopy based on detail core investigation.The fenestrae pore and shelter pore cemented residual in sediment to early diagenetic stage are the main pore type of algal limestone in lower Yijiangfang formation,micro-pores presented inter-crystalline of euhedral calcite are mainly distributed in algal particles.The formation of micro-pores could be controlled by recrystallization and original organic matters including its evolution.The moldic pores of biological shells and internal pores of aggregates composed by silica beads and euhedral calcite crystal are the main type pores of boundstone in upper Yijiangfang formation.Micro-pores are showed in organism framework and inter-crystalline of recrystallized calcite and crystal face of euhedral quartz in matrix.The alteration of hydrothermal fluid in early diagenetic stage should be the main cause of pores and micro pores in ST1 well.The preservation of pore and micro pores spaces may be linked with liquid hydrocarbon charging as the absence of a large amount of bitum.The development of different types carbonate reservoirs in the study area is controlled by strike-slip faults,lithofacies and seal of cap rocks.The late Caledonian-early Hercynian is the main active period of NE-NNE strike-slip faults,which controlled the associated hydrothermal fluid activity.The ascending hydrothermal fluid permeated the surrounding rocks and altered different types carbonate.The difference in lithology and texture of carbonate with fluid-rock interaction may be resulted in different types carbonate reservoir and different reservoir space.In addition,the intertidal-tidal sedimentary facies belt of restricted platform is conducive to the development of microorganisms and algae.The related mineral stabilization process and the thermal evolution of primitive organic matter are important reasons for the formation and preservation of pores and micro pores in limestones.The seal of ulta-thick fine-grained clastic rocks in upper Ordovican provides suitable conditions for confinement of the hydrothermal activity and fluid-rock interaction and hydrocarbon accumulation.