Understanding Tight Gas Sandstone Reservoirs from High-Definition Oil-Based Microresistivity Image Logs
Case study from Tarim Basin
The thick tight sandstones in ten slim-hole vertical wells were classified to different paleo-current energy deposits by new generation high-definition oil-based microresistivity image logs. The thick tight sandstones were 100 to 300m thick in the Cretaceous Bashijiqike Formation in Tarim basin. The tight sandstones deposited under different energy sedimentary conditions correspond to different reservoir quality. High-current energy sandstone deposits are commonly comprised of good sorting and roundness, which could be expected to have good reservoir quality and good gas production performance. The core data from the wells, however, shows that the tight sandstones deposited under high paleo-current energy condition produced low gas production. The image logs were also used to identify open natural fractures that impact gas production performance in the tight gas sandstone reservoirs. Conventional open-hole logs were not able to detect the sedimentary features and the natural fractures.
The high-definition oil-based microresistivity image logs make it possible to identify the different kinds of the tight sandstones by sedimentary structures and textures related to paleo-current energy, planar parallel lamination, flaser bedding, cross-stratification, graded bedding, slump bedding, mudchip clast, sandstone conglomerate, etc. The sediment characteristics successively correspond to paleo-current energy from low to high, which represent sediment supply sources from distal to proximal. The tight sandstones could have similar porosity but largely different permeability, and completely different reservoir quality. Open natural fractures further enhance large permeability changes in the tight sandstones. The open fractures were identified by the integration of the resistivity images and standoff images converted from inversion processing. By considering core data and field-scale geologic settings under sublacustrine fan delta front and braided delta front depositional environments, the sedimentary structures and textures revealed the major geologic controls to possible tight gas sandstone reservoirs. The core data verified the sedimentary structures and textures on the high-definition images, the gas production data validated the tight gas sandstone reservoir qualities identified from the sedimentary structures and textures.
The case study demonstrates the interpretation of the sedimentary structures and textures from the high-definition image logs, and meanwhile presents the identification of open natural fractures on the resistivity images and standoff images. The sediment characteristics and open fractures must be considered in the tight gas sandstone reservoir evaluation. It develops the comprehensive understanding of the major geologic controls that differentiate the tight sandstones associated with different current energy and different reservoir quality. In the slim-hole wells, the high-definition oil-based microresistivity image logs provided a new solution to predict possible tight gas sandstone reservoirs from different paleo-current energy deposits in the Cretaceous Bashijiqike Formation.