In the Quest of Open Fractures in the Crystalline Basement of the Malay and Penyu Basins

Abstract

Coherent rock may fracture naturally into four predictable orientations/attitudes with respect to the generating stress system. Thus formed, fractures parallel and perpendicular to the maximum principal stress are potentially open features. Area and region-wise, this maximum stress direction is commonly horizontal (SHmax). Slip on the two shear fractures may further generate second-order fractures, with similarly predictable orientations/attitudes. Fractures are the major contributors of secondary porosity in crystalline rocks. The crystalline basement of the Malay and Penyu basins consists of metamorphic rocks as well as occasional pre-Tertiary igneous bodies. These rock types are analogous to those outcropping in the Eastern Belt of the Peninsula. Extensive field studies throughout many years have established that these rocks were subjected to two (for the lower Mesozoic) and up to four multiple (Carboniferous) tectonic deformations. Each of the deformations may have resulted from differently orientated stress systems and consequently their respective fracture characters may have become degraded or destroyed. At regional scale, radar satellite imagery and aerial photographs show preferred fracture orientations that are consistent with pre-Tertiary stress systems. For instance, initially open fracture directions in lower Mesozoic-upper Palaeozoic basement correspond with Cretaceous dolerite dykes. The responsible SHmax direction was ENE-WSW for this particular case. Regional fractures (most probably all are faults because of their several-kilometre-long dimensions) in the Malay and Penyu basins also show preferred orientations that are consistent with those mapped onshore (figure 1). Outcropping “crystalline basement” rocks in the Eastern Belt display a variety of fracture orientation as well as fracture character. Differences with those implied by the regional lineament patterns could often be determined as being of local nature, such as being situated within a shear zone, being associated with the intrusive form of an igneous body, or resulting from decreasing overburden, and so forth. Nevertheless, some useful relationships between lithology, rock texture, and fracture density were established (figure 2). Tertiary SHmax orientations in the Malay and Penyu basins were initially determined using caliper logs. This was strongly supported by subsequent well-bore breakout studies using images, such as FMI. Most of the Malay Basin is governed by N-S SHmax (figure 3). A belt following the Western Hinge Line on the western side of the Malay Basin has SHmax that ranges between Northwest and West-Northwest . The Penyu Basin is under the influence of SHmax orientated East-West (figure 4). The regional fault pattern in these basins have preferred orientations consistent with the SHmax orientations (compare with figure 1). Finally, we suggest that open fracture directions in the basement of the Malay and Penyu basins are both parallel and perpendicular to the SHmax directions as indicated for the respective areas in figure 4. These fracture directions provide fluid pathways; in addition the proximal fracture environments of the basement are most likely more favourably charged by fluids compared with those remote from the structures.