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 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.

Ratana Field, Potwar Fold Belt, Northern Pakistan: High Intensity Fracture Zones Related to Major Thrust Faults as Revealed by Seismic Fracture Prediction

“…Similar reservoirs are common elsewhere in the Potwar fold-and-thrust belt (Jadoon et al, 2003;Jadoon et al, 2007;Hasany et al, 2019) and fracture permeability is reported to attain 5 D in some fields. In these types of reservoirs, it is necessary to have a high fracture intensity or high fracture porosity for the reservoir to be economically viable.…”

“…In the Wargal Formation in Ratana-4, a total of 54 steeply inclined open-mode fractures were recorded over an interval of ~120 m, although most of these were developed in a cleaner limestone interval which was only ~30 m thick and are therefore likely to be part of a separate fracture system. Some constraints on the fracture networks and their timing can be gained from studies of related outcrops and adjacent fields (Guilhaumou et al, 2004;Jadoon et al, 2007;Ali-Shah et al, 2008;Dasti et al, 2018). The Sakesar and Chorgali Formations are exposed in Chorgali Pass, some 30 km west of the Ratana field, where the steeply dipping Khari-Murat thrust brings these carbonate units to the surface from a depth of 5 km in a very short lateral distance (Kamran and Siddiqi, 2011; see Fig.…”

“…However, there have been few attempts to predict fracture distribution in producing fields; moreover, some studies have failed to recognise the significance of fractures (Khalid et al, 2016;Ahmed et al, 2018). Many wells drilled on the crests of structures adjacent to productive wells are reported to have poor production because an effective fracture network has not been developed in the drilled location (Jadoon et al, 2007). Recent studies suggest that the fracture play in the fold-and-thrust belt may be valid even in the absence of a structural closure (Hasany et al, 2019).…”

An Integrated Approach to Evaluate Naturally Fractured Carbonate Reservoir in TAL Block, Kohat Basin

“…Strata-bound [2] type of fractures are present in this facie. It consists of argillaceous limestone with well developed bedding features.…”

Estimation of the Shale Oil/Gas Potential of a Paleocene–Eocene Succession: A Case Study from the Meyal Area, Potwar Basin, Pakistan

“…The phenomenon of fracturing is related with the folding of the Meyal anticlinal structure given in Fig. 16, proposed by Jadoon et al (2005).…”