Abstract

Fracture is of primary importance to gas producibility from many tight sandstone reservoirs in the Kuqa depression, Tarim Basin, NW China, but the orientation, size and plane porosity of fractures in the subsurface is difficult to measure directly. Terrestrial light detection and ranging (LIDAR) surveys can offer factual information of outcrop-based research efforts to characterize fracture development laws and control factors. In this paper, we obtain the three-dimensional (3-D) point cloud data by the LIDAR survey from a typical outcrop. Matching with high-resolution digital photos and artificial measured information, the 3-D positions of natural fractures are extracted strictly in the data volume section. Furthermore, the fracture patterns and controlling factors are revealed accurately with the fracture and reservoir model founded by computer simulation technology based on the systematic sampling and laboratory analysis. It is founded that three groups of shear fractures are mainly developed with large inclination, short trace length and small spacing of normal distribution. The fracture aperture is related to the orientation with bimodal distribution of 0.2~0.4mm and 0.8~1mm. The fracture patterns provide the literal distribution of grid-like fracture zone and reticular fracture zone with a single and a double set of advantage orientation respectively. The fractures in the former zone have greater trace length, spacing and smaller surface density and run through the adjacent shale mostly while the latter is just the opposite. The plane porosity of fractures in 11 single sandbodies of different microfacies has been calculated quantitatively with a range of 0.026~0.081% and an average of 0.05%. It is in the sandbodies of underwater distributary channel is better than mouth bar and distal bar. It turned out that the size of fracture is controlled by lithology, bed thickness, maximum paleostress and rock composition with a good exponential relationship. The fractures are more developed relatively with the coarser particle size of the rock, the thinner thickness, the greater paleostress, the lower content of feldspar and the higher content of cement.

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