Subsurface Fracture Analysis in Carbonate Reservoirs: Kohat/Potwar Plateau, North Pakistan

ABSTRACT Carbonate reservoirs in Northern Pakistan are characterized by tight limestone. In these reservoirs, fractures are important for production and reservoir modeling. This paper addresses problems related to subsurface fracture analysis based mainly on image logs. Natural fractures occur as systematic and unsystematic sets of definite and random orientation respectively. The subsurface analysis of fractures uses electrical and acoustic image logs to characterize fractures as either natural or induced features. They are classified as conductive or resistive features, representing possibly open or closed (mineralized) fractures, respectively. Using image logs, natural fractures are interpreted and classified descriptively to be continuous or discontinuous features representing systematic fractures or classified as chicken-wire (microfractures) fractures representing unsystematic sets. Statistical analysis of fractures is used to classify them into geometrical and genetic sets as longitudinal (extensional), transverse (tensional), and oblique (shear) to the structure. Transverse fractures are known generally as most open. They develop parallel to the maximum horizontal in-situ stress and extend deep into the structure. Longitudinal fractures, those parallel to the fold axes, are observed to produce hydrocarbons in several fields in Northern Pakistan. Fracture density impacts production and reserves calculations. However, fracture density is strongly influenced by the lithology and layer thickness. Widely spaced fractures are observed in massive carbonate reservoirs, and closely spaced fractures of narrower aperture are observed in laminated strata. Thus, individual fractures in massive carbonates require to be identified for their impact on production. Fractures are observed to occur as discontinuous features of right or left-stepping geometry and as en echelon features of significantly wider aperture in shear bands. These features together with vugs and leached features may provide zones of higher porosity, permeability, and storage capacity with isolated distribution in tight carbonates. Therefore, knowledge about fracture occurrence and distribution is important to predict sweet spots for drilling and field development.