Geology and Petroleum Potential of the Makran Coast, Pakistan
Authors: Harms J.C. , Cappel H.N. , Francis D.C. 
Published: December 2007

Coastal Makran is generally acknowledged as a subduction zone where the Indian Ocean plate moves northward under continental crust. The geologic history deduced from facies and structures is complex and can be divided into three major episodes :A Middle Miocene and older phase dominated by turbidity currentdeposition on a vast deep-sea fan which originated from the east, presumably supplied from uplands raised by the collision of India and Asia. Sandstones are fine- to coarse-grained, conglomeratic in part, quartzose, and contain shallow-water megafossils. Sediment transport was towards the west and sandstones found along an axial distance ofnearly 400 km show lime change in texture or bedding characteristics, suggesting a fan rivaling the dimensions of the Bengal Fan.A Late Miocene to mid-Pleistocene phase of slope, shelf, and coastal plain progradation. Seaward advance of this sedimentary prism was accomplished incrementally by several generations of shelf-slope lobes, measured in dimensions of 10's of kilometers and supplied by local drainages from northern coastal ranges. The land and shelf configuration probably resembled that of the present costal area. Tectonically, these sediment lobes were soon folded and uplifted on their landward margin, thus cannibalized by erosion to provide part of the sediment for the next younger lobe. The structure is dominated by recumbent anticlines with axial planes dipping northward and broad asymmetric, elliptical synclines.A mid-Pleistocene to modem phase of dramatic uplift and local normal faulting. Very young shoreline sediments in some areas have been raised 500 m, and normal faults with substantial throw have been mapped seismically onshore and offshore. Tensional motion has been identified on some earthquake data and may be related to uplift.

Spectacular mud volcanoes have been built in several areas along the Makran Coast by gas charged water escaping to the surface. The gases are mainly methane, but traces of heavier hydrocarbons and isotopic compositions indicate generation from thermally mature source rocks. Possible reservoirs include Middle Miocene coarse-grained turbidites or Late Miocene-Pliocene shelf sandstones, though the latter are mostly very fine grained, burrowed, and low in permeability.

Attractive structures are defined particularly offshore by prominent seismic reflectors thought to be riddle Miocene. However, these potential objectives are overlain by overpressured young sediments that are serious obstacles to drilling.

Introduction

Makran comprises the southern part of Pakistan and Iran between Sonmiani Bay near Karachi and the Straits of Hormuz. Viewed on a regional scale, it is a great festoon of folded and faulted Tertiary sediments extending 800 km from the Las Bela axial fold belt on the east to the Oman Line on the west (Figure 1). It is well known that these eastern and western boundaries separate Makran from older terranes with deformational styles and histories distinct from that of Makran. India moved northward in its collisional course with Asia along a transform fault expressed as the Owen fracture zone, the Murray Ridge, and the axial fold belt of Pakistan. The Arabian and Iranian plates converged to form the Zagros ranges. In both cases, blocks of continental crust have moved northward against other continental plates, closing and crushing former deep oceanic seaways. Along Makran, it appears that oceanic crust has moved northward, subducted beneath a continental margin composed of small plates and complex ophiolite zones. A recent review of regional tectonics is included within a volume on the geodynamics of Pakistan (Farah and DeJong, 1979).

Coastal Make-an and the area to the north is an accretionary wedge of deformed sediments ranging in age from perhaps Late Cretaceous to Recent, piled up at an oceanic subduction margin. The structure and depositional setting has been compared to "a typical arc model" composed of upper-slope deposits followed by lower-slope and trench deposits, progressively deformed by continuing subduction (Farhoudi and Karig, 1977). However, as an arc-trench system, Make-an is hardly typical, indeed it is perhaps largely anomalous in its characteristics, as pointed out by perhaps largely anomalous in its characteristics, as pointed out by Jacob and Quittmeyer(1973). The arc-trench gap is on the order of 500 km, far wider than most systems. A possible Benioff zone is weakly developed. Shallow focal mechanism solutions indicate instances of tension in the oceanic slab. Volcanic centers are widely spaced along the arc feature. Additionally, a very large pan of the accretionary prism is exposed, and a significant volume of post-Middle Miocene sediments are shallow shelf deposits, not trench post-Middle Miocene sediments are shallow shelf deposits, not trench and slope deposits as alleged by Farhoudi and karig (1977).

This paper reports the results of intensive field mapping, reflection seismic studies, and drilling conducted along the Pakistan Makran coast between 1973 and 1977. Detailed studies were concentrated within 60 km landward of the modern coastline and across the shelf to water depths of 200 m. It is within this belt that sedimentary strata and structural development seemed most appropriate for petroleum exploration. Although no commercial discoveries of petroleum exploration. Although no commercial discoveries of hydrocarbons have been made, some potential remains. Whether or not discoveries are ever made, Coastal Makran provides a valuable and perhaps unique opportunity to study a well exposed and young acretionary margin.