Origin, distribution and implications on production of bedding-parallel fractures: A case study from the Carboniferous KT-Ⅰ Formation in the NT oilfield, Precaspian Basin, Kazakhstan

Abstract

Bedding-parallel fractures (BPFs) are the breakage of rocks along weakness bedding plane of stratification or foliation. At present, the research of fractures is primarily focused on high-angle structural fractures (SFs), but few systematic efforts have been made on the formation mechanism, distribution and influences on oil production of BPFs. Knowledge of attributes of BPFs in carbonate reservoirs is necessary for well-site design and reservoir waterflooding development.Observation and description of cores and image logging and laboratory measurements including cathodoluminescence (CL), fluid inclusion, acoustic emission experiments, porosity, permeability and well testing data were used to gain insight into the formation mechanism, controlling factors and impacts on oil production of BPFs. The results show that the length and aperture of BPFs range from centimeters to tens of meters and from 30 μm to 100 μm respectively. BPFs are primarily shear fractures without cements in the study area. There are 3 periods of SFs in the study area and the formation timing of BPFs and the SFs of the 2nd period are simultaneous. The origin of BPFs is related to the development of thrust faults and folds. Shear BPFs were the result of interlayer sliding during the formation of anticline and thrusts under the effect of tectonic compaction. Extension BPFs formed because of the relative movement of the hanging wall and footwall of the thrust faults. Shear BPFs are more common than extension BPFs because both anticline and thrusts can result in the formation of shear BPFs. Many factors can govern the development of BPFs including structural position, rock composition and structure, distance to faults and bed thickness. The formation of BPFs is promoted by low structural position, rapid vertical changes of the rock composition, small grain size, close distances to faults and thin bed thickness. Although the aperture of low-angle BPFs is smaller than that of fractures at high angle to bedding due to the influence of overburden pressure, the low-angle BPFs can partially improve the reservoir permeability and partly enhance the initial production of individual wells.