Fracture Characterization and Sweet Spot Mapping in Naturally Fractured Tight Carbonate Reservoir, Opening a New Play in Lower Indus Basin, Pakistan

Abstract

Chiltan Limestone of Jurassic age is widely developed in Lower Indus Basin of Pakistan. The formation was deposited in the shelf-lagoon setting with wackestone-packstone as the dominant microfacies. Calcite cementation and intense compaction drastically reduced the matrix porosity (avg. 1.5%) and permeability (<0.01 mD) making the formation a tight reservoir in the study area. Natural fractures play an important role in hydrocarbon production from such tight carbonate reservoir, hence, knowledge of the exact positions of fractures is critical for precise placement of wells for better production. In order to characterize and map the natural fractures, core and image logs along with 3D seismic data were used. Natural fractures were detected and studied on image logs at a fine scale, which revealed that these fractures exist in clusters, often called Fracture Corridors (FC). These FCs are mostly associated with minor faults, not visible on seismic data due to their subtle nature. However, by using seismic attributes such as variance, dip deviation and ant-tracking, these can be easily mapped. These attributes enhance the structural discontinuities by correlation of seismic amplitudes of adjacent traces, which helps in prediction of subsurface positions and lateral extents of these FCs. Several attributes were extracted from the seismic datasets of pre-stack time and depth migrated volumes to map FCs, results were verified and calibrated with image log data. Regional and local in-situ stress directions and structure patterns in the study area were important to interpret and understand the fracture patterns. This led to building a subsurface geological model, which revealed that the fractures are present all over the field area. Based on the results of this study, horizontal drilling in the top part of the naturally fractured reservoir was proposed. Well path was designed to intersect a maximum number of critically stressed FCs, considered as zones of better permeability in the tight reservoirs. Drilling of the horizontal well validated the subsurface geological model of FCs and resulted in obtaining better hydrocarbon production from a Tight Gas reservoir. This is the first well in the area that was successfully tested to confirm the commercial flow of hydrocarbon from Tight Chiltan Limestone. This is a major step that has confirmed a new play of Jurassic Chiltan Limestone, thus creating a new opening for exploitation in the matured Lower Indus Basin of Pakistan.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018