Filling Of The Fracture-cavity Space In Ordovician Karst Reservoirs Of The Tahe Oilfield And Its Reservoirs Petrophysics

As complicated distribution of fracture-cavity and its filling formed by karstification for more than 100 Ma in middle Paleozoic, heterogeneity of fracture-cavity reservoir in the Tahe Oilfield is extremely strong, and different filling types and their forming process control the reservoir spaces. Clastic sediments, breakdown breccia and chemical fillings is the most important type of fillings in karst fracture-cavity. Different water hydrodynamic conditions form different sedimentary facies and units genetic in three karst zones, while studying on sediment deposition mechanism of fracture-cavity seam based on buried clastic sediments faces many problems. What’s more, defining genetic types of chemical filling and specifying geochemical characteristics of different genetic types, as well as their formation process, is the basis of theory to find out the origin of karst fracture-cavity reservoir spaces.By investigations of modern and ancient karsts, and comprehensive interpretations on geological, well-logging and seismic data in the studied area, the author analyzed comparatively the sedimentary characteristics and sedimentary facies of cave sediments in three karst zones, and then carried out mineral composition and grain size analysis on clastic sediments on the basis of thin sections observation, inclusions homogenization temperature, trace elements and rare earth elements testing data, and indicated hydrodynamic conditions and paleo-redox conditions of the water when sediments was deposited and provenance studies, further studying on the diagenesis of cave sediments in different karst zones.Calcite is given priority to in Tahe Ordovician fracture-cave chemical fillings, which can be divided into calcite cement between clastics and collapse breccia, recrystallization calcite cement in micro cracks of limestone and fracture-cave giant crystal calcite.Systematic sampling on the different calcite of three karst zones, based on photomicrograph study of calcite in plane-polarized light and cathodoluminescence, and then carried out geochemical analysis, including inclusion, trace element, rare earth element, and carbon, oxygen, strontium isotopes.with different occurrence calcite. Comprehensive analysis of calcite occurrence and the geochemical indicators, Tahe Ordovician fracture-cave calcite can be divided into three genetic types: freshwater calcite in supergene environment, calcite in normal burial environment and hydrothermal calcite, and then points out different calcite geochemical features and distribution rules.Focusing on deposition mechanism and diagenesis, chemical filling mechanism, the origin of reservoir spaces in karst fracture-cavity is analyzed, indicating that fillings types and diagenesis have a significant impact on fillings physical property.And then, based on the filling type, using the measured core data and conventional logging, carried out multiple regression analysis to establish different filling types’ logging interpretation models of porosity and permeability. This interpretation model is proved scientific, rational, and practical when applied to 146 wells in 4th, 6th and 7th area of Tahe Oilfield. This paper reaches some conclusions: Collapsed breccia and coarse mechanical sediment have good porosity and permeability, which is in keeping with well’s relatively high production; The small caves of less than 0.5m height have better physical property when compared to fractures although fractures improves reservoir’s physical property obviously. This model provides a solid foundation for further geological modeling and the efficient exploitation of carbonate karst reservoir.