A Fault-Controlled Non-Exposed Meteoric Karstification:A Case Study of Ordovician Carbonate Reservoir at Structure YM2 in Northern Tarim Basin,Northwestern China

Karst reservoir takes an important role in carbonate reservoir,but it had been long regarded the leading reason that eogenetic carbonatic rock,after lifted to the ground,suffers phreatic water table-controlled karstification.Based on the understanding of geological setting of the YM2 area in the northern Tarim uplift and integrated analysis of lithology,geochemistry and the controlling factors of karstification,this study suggests that karstification may have resulted in the formation fracture-cavern reservoir in the areas with the continued depositional sequence.The calcites of various structures and textures were studied in detail to understand the properties and provenance of diagenetic fluids using a series of geochemical analyses such as C,O and Sr isotopes,REE,electron microprobe,cathode-ray luminescence,and fluid inclusion,in combination to observation of core and thin sections.Meteoric water is regarded to be the main karst fluid responsible for the formation of porosities,whereas deep hydrothermal fluid just modified reservoirs at late stage.Analysis of the features and development of strike-slip faults in the YM 2 shows that the structure forming and strike-slip fault development provide prerequisite for karsitification.Furthermore,a new meteoric karstification controlled by steep structure and faulting is proposed,in which carbonatic rock sequences,although covered by thick clastic strata,is uplifted at the late stage,accompanied with development of strike-slip faulting,a large amount of fractures is formed within the top of carbonatic rock sequence,and meteoric water penetrates along and results in karstification within fractures,thus forming large-scale fracture-cavern reservoirs.This model has broken through the restriction of the exposure-type buried hill karst model,improved our understanding of karst reservoir formation in the areas with whole carbonatic rock sequence,and finally broadened our thought of oil and gas exploration in karst reservoirs.