Abstract

Carbonate reservoirs in Northern Pakistan are characterized by tight limestone.In these reservoirs, fractures are important for production and reservoirmodeling. This paper addresses problems related to subsurface fracture analysisbased mainly on image logs.

Natural fractures occur as systematic and unsystematic sets of definite andrandom orientation respectively. The subsurface analysis of fractures useselectrical and acoustic image logs to characterize fractures as either naturalor 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 classifieddescriptively to be continuous or discontinuous features representingsystematic fractures or classified as chicken-wire (microfractures) fracturesrepresenting unsystematic sets. Statistical analysis of fractures is used toclassify them into geometrical and genetic sets as longitudinal (extensional), transverse (tensional), and oblique (shear) to the structure. Transversefractures are known generally as most open. They develop parallel to themaximum horizontal in-situ stress and extend deep into the structure.Longitudinal fractures, those parallel to the fold axes, are observed toproduce hydrocarbons in several fields in Northern Pakistan. Fracture densityimpacts production and reserves calculations. However, fracture density isstrongly influenced by the lithology and layer thickness. Widely spacedfractures are observed in massive carbonate reservoirs, and closely spacedfractures of narrower aperture are observed in laminated strata. Thus, individual fractures in massive carbonates require to be identified for theirimpact on production. Fractures are observed to occur as discontinuous featuresof right- or left-stepping geometry and as en echelon features of significantlywider aperture in shear bands. These features together with vugs and leachedfeatures may provide zones of higher porosity, permeability, and storagecapacity with isolated distribution in tight carbonates. Therefore, knowledgeabout fracture occurrence and distribution is important to predict sweet spotsfor drilling and field development.

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