FU Xunxun,
ZHAO Weibo,
PEI Wenchao,
ZHAO Xiaohui,
SHI Lei,
DONG Guodong,
WANG Huaichang,
LIU Xiaopeng,
LU Zixin,
YIN Liangliang,
GAO Xing,
CHEN Juanping
, Available online , doi: 10.11932/karst2024y040
Abstract:
In 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 has been discovered in the northern terrigenous clastic rock strata. The southern limestone strata has a wide distribution, covering approximately 14 square kilometers, and exhibits considerable thickness, ranging from 5 to 30 meters.However, the limestone reservoir is dense with an average porosity of 2.1%, an average permeability of 0.21mD, and an average throat radius of 0.12μm. For a long time, exploration had not focused on it due to its dense reservoir lithology.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 relate to these structural development.To solve the above difficulties, the following steps have been studied and adopted in this study.Considering the tectonic setting,the structural fracture characteristics the of the Taiyuan limestone were firstly analyzed by field profiles, core observations, and well log data.Secondly the impact of structural fractures on the porosity of dense limestone reservoirs was observed by uniaxial loading rock core nuclear magnetic resonance (NMR) technology and CT scanning for the first time. Furthermore, the fluid inclusion was investigated to analyze the conducing role of the fracture networks during gas accumulation process.Lastly, based on the work mentioned above,to predict the development of structural fractures and favorable blocks exploration, an improved Gaussian curvature method (IGC) was proposed,according to the specific geological conditions, and it was applied in well deployment in the Hengshan area.The results show that: 1) In the study area, regional tectonic stress field is mainly influenced by the Yanshanian Movement,and the main tectonic framework was mainly established by it with NW-SE compressive stress.Compared with low-angle fractures and oblique fractures, high-angle tension fractures have low filling levels and good effectiveness.it often penetrates, cuts, and dislocates other types of fracture improving the limestone reservoir properties. In addition, the formed fracture network system may facilitate the efficient migration of hydrocarbons; 2) Rock physics experiments had manifested that structural fractures greatly contribute to the physical properties of the tight limestone reservoirs.With the development of fractures, the average porosity doubled from 2.1% to 4.2%. The contribution rate of fractures to total porosity ranged from 14.3%~72.7%, with an average of 43.4%. Among the three reservoir rock types, the porosity of powder crystal limestone increased the most, followed by algae bound limestone, and mud crystal limestone had the smallest increase; Combined with typical well reservoir-forming temperature, burial history, and thermal evolution history in the study area,inclusion analysis made clear that the widely developed fracture networks formed an effective transport system, and hydrocarbons effectively migrated by it during the main reservoir-forming period from the Late Jurassic to the Early Cretaceous.That is to say, generated from the main coal-bearing source rocks 8# coal and 5# coal located at the top and bottom of the limestone reservoir,the natural gas entered the limestone reservoir by the effective transport system.And supplemented by a cap rock of the marine mudstone on the top of the Taiyuan Formation, a superior "sandwich" type reservoir formed;4) The improved Gaussian curvature method (IGC) can predict the development degree of favorable structural fractures. The higher the IGC value, the more developed the high-angle tension fractures, and the more favorable the conditions for natural gas enrichment.This method was a debut in the Hongshan area to support well deployment. YT1H*,the first risk exploration well, achieved a gas production of 540,000 cubic meters per day, and marked a breakthrough in the exploration of the Taiyuan limestone in the Ordos Basin. In the past 2 years, 9 industrial gas wells have been discovered in this area, 5 of which have output above 100,000 cubic meters per day. Among them, YT4H* achieved an output of 1,088,000 cubic meters per day.The above exploration achievement demonstrated the effectiveness of the research and provided a reference for re reacquainting tight carbonate reservoirs. It is expected that this study can further guide the large-scale exploration and development of the Taiyuan Formation limestone in the Ordos Basin.
