Geological significance,prediction and application of structural fractures in compact limestone reservoirs of the Taiyuan Formation,the Ordos Basin

FU Xunxun; FAN Liyong; PEI Wenchao; ZHAO Xiaohui; SHI Lei; DONG Guodong; WANG Huaichang; LIU Xiaopeng; LU Zixing; YIN Liangliang; GAO Xing; CHEN Juanping

doi:10.11932/karst2024y040

Abstract：In the Ordos Basin,the Taiyuan Formation mainly consists of a marine-terrestrial transitional sedimentary system.The northern part featured shallow water delta deposits dominated by terrigenous clastic rocks,while the southern part consists of coastal marine limestone deposits.The Shenmu gas field was discovered in the northern terrigenous clastic rock strata.The southern limestone strata have a wide distribution,covering approximately 14 square kilometers,and exhibit considerable thickness,ranging from 5 to 30 meters.However,the limestone reservoir is compact with an average porosity of 2.1％,an average permeability of 0.21 mD,and an average throat radius of 0.12μm.The exploration of this reservoir has not been fully acknowledged for an extended period.Recent years,further research has indicated that the high-yield gas wells are often related to the development degree of structural fractures.However,there is a lack of systematic analysis of whether the quality of the limestone reservoir and the natural gas enrichment relating to the structural development. To overcome the above difficulties,the following steps have been studied and adopted in this study.Considering the tectonic setting,the structural fracture characteristics of the Taiyuan limestone were firstly analyzed by means of field profiles,core observations,and well log data.Secondly,the impact of structural fractures on the porosity of compact limestone reservoirs was observed by uniaxial loading rock core Nuclear Magnetic Resonance(NMR)technology and CT scanning for the first time.Furthermore,fluid inclusions were investigated to analyze the conductive role of the fracture networks during the gas accumulation process.Lastly,based on the aforementioned work,an Improved Gaussian Curvature method(IGC)was proposed to predict the development of structural fractures and identify favorable exploration blocks,tailored to the specific geological conditions.This method was subsequently applied in well deployment in the Hengshan area. The results show,(1)In the study area,regional tectonic stress field is mainly influenced by the Yanshanian Movement,which has developed NW-SE compressive stress.The tectonic stress field in the Yanshanian Period established the main tectonic framework of the study area.Compared with low-angle fractures and oblique fractures,high-angle tension fractures have lower degree of filling and better effectiveness.They often penetrate,cut,and displace other types of fractures,thus improving the limestone reservoir properties.In addition,the fracture network system formed may facilitate the efficient migration of hydrocarbons.(2)Physics experiments on rocks have manifested that structural fractures significantly influence the physical properties of compact limestone reservoirs.The average porosity doubled from 2.1％to 4.2％due to the development of fractures.The contribution of fractures to total porosity ranged from 14.3％to 72.7％,with an average of 43.4％.Among the three types of reservoir rocks,powder crystal limestone exhibited the greatest increase in porosity,followed by algae-bound limestone,while mud crystal limestone showed the smallest increase.(3)By combining fluid inclusion analysis with typical well reservoir-forming temperatures,burial history,and thermal evolution history in the study area,this study has drawn a conclusion that the extensively developed fracture networks have created an effective conductive system.During the main reservoir-forming period from the Late Jurassic to the Early Cretaceous,hydrocarbons were effectively transported through the fracture network system.The main coal-bearing source rocks,namely,# Seam No.8 and Seam No.5 located at the top and the bottom of limestone reservoir,generated natural gas that migrated into the limestone reservoir through the fault system for accumulation.With the direct seal provided by the marine mudstone at the top of the Taiyuan Formation,an excellent"sandwich"-style hydrocarbon accumulation has formed.(4)IGC can effectively predict the degree of development of favorable structural fractures.A higher IGC value indicates a greater development of high-angle tension fractures,which can create more favorable conditions for natural gas enrichment. This method was first applied in the Hongshan area to support the layout of wells.The first well for risk exploration,Well YT1H*,achieved a gas production of 540,000 cubic meters per day,realizing a breakthrough in the exploration of the limestone in the Taiyuan Formation in the Ordos Basin.In the past two years,a total of nine industrial gas wells have been successively discovered in the study area.Five of these wells produced more than 100,000 cubic meters per day.Among them,Well YT4H*achieved an output of 1.088 million cubic meters per day.The above exploration results demonstrate the effectiveness of this study,providing a reference for re-evaluating compact carbonate reservoirs.It is expected that this study can further guide the large-scale exploration and development of the limestone in Taiyuan Formation in the Ordos Basin.