Hydrocarbon Exploration in the NW Himalayas: A Perspective from Structural Geology
Dilip K. Mukhopadhyay
Department of Earth Sciences, Indian Institute of Technology, [email protected]
The metamorphic core complex in the axial zone of the arcuate Himalayan Mountain belt, known as the Central Crystallines, is flanked by two sedimentary belts (Fig. 1). Both source and reservoir rocks are present in these two sedimentary belts. During the last 125 years several oil/gas fields have been discovered in the Himalayan foreland in Pakistan and in Assam-Arakan foreland in NE India. But surprisingly, in the intervening areas of India, Nepal and Bhutan Himalayas the exploration for hydrocarbon has not been successful as yet although many oil and gas seeps are known from these areas since historical times, particularly in the NW Himalayas. An important reason for this lack of success could be that that the structural evolution of the Himalayan sedimentary belts was not very well understood during the exploration activities in the 50s and 60s. With renewed interest in exploration for hydrocarbons in the Himalayas, it is imperative that the structural complexities and evolution be well understood for further exploration work.
A continuous sequence of Proterozoic to Cretaceous/Palaeocene sedimentary rocks, collectively known as Tethyan Sedimentary Zone (TSZ) overlies the Central Crystallines towards north. The contact between Central Crystallines and TSZ is a low angle normal fault, which probably represents a reactivated thrust fault. South of the Central Crystallines, two sedimentary lithotectonic zones are traditionally recognized, viz., Lesser Himalaya Zone (LHZ) with a sequence of Proterozoic to Lower Cambrian age and Sub-Himalaya Zone (SHZ) consisting of Tertiary rock sequences deposited (except for the lower part) subsequent to the India-Asia collision. The Main Central Thrust (MCT) separates LHZ from Central Crystallines and the contact between LHZ and SHZ marks the Main Boundary Thrust (MBT). The MCT and MBT are supposed to be two longitudinally continuous intracontinental boundary thrusts in the Himalaya. The SHZ overrides the Indo-Gangetic Alluvial plain along the Main Frontal Thrust (MFT). The MCT, MBT and MFT are thought to be splays from a gentle northerly-dipping detachment. All the rock sequences/lithotectonic zones are very well developed and preserved in the NW Himalayas in the Indian states of Jammu and Kashmir, Himachal Pradesh and Uttarakhand.
The northern sedimentary belt (TSZ) consists primarily of limestones with subordinate amount of sandstones and shales. The rocks are thinly bedded with thickness of individual beds ranging in scale from a few mm through few cm to tens of cm. The well-bedded rocks trace spectacular folds in outcrop to intermediate scales. Folds, tens of meters in amplitude, are seen on subvertical gorges cut at high angles to strike. Tightness of folds vary from very open to very tight. The folds have straighter limbs and sharp hinges of limited areal extent giving rise to chevron or kink fold geometry. Axial planes are usually steeply dipping to subvertical and hinges are horizontal or gently plunging. Thus the folds usually have horizontal upright geometry. Evidence of layer-parallel slip is very common with the slip direction towards the hinges of mesoscopic folds. The folds have developed through buckling of thinly-bedded multilayer with high viscosity contrast and moderate to low packing distance. Consideration of map pattern, stratigraphic relations and analysis of structural data show that the large-scale structure can be best described as a horizontal upright syncline. The innumerable faults seen in outcrop to intermediate scales are accommodation faults developed during folding. This interpretation is in contradistinction to a few recent works that suggest the deformation pattern in TSZ is akin to foreland fold-thrust belts.
The southern sedimentary belt consisting of SHZ and LHZ shows large number of thrust faults with the fault traces parallel to the mountain range. Although this terrain has high vegetation cover, the thrust faults are easily recognized through reversal of stratigraphy and appropriate fault rocks. A number of anticlines and synclines are present whose axial traces are parallel to the traces of thrust faults suggesting that they are genetically related, i.e., the folds are related to fault displacements. The dip domain data show that these folds have approximately chevron or kink fold geometry. In recent years a number of balanced cross sections have been constructed across this sedimentary belt. These sections convincingly show that the structural styles in this belt conform to those found in foreland fold-thrust belts in many tectonic provinces in the world. This is in contrast to the large-scale synclinorium seen in the northern Tethyan sedimentary sequences. Further, the buckle folds in the TSZ are most likely a result of variable slip on detachment, where as the folds in the SHZ and LHZ are typically fault-bend or fault-propagation folds.
The large syncline in the TSZ apparently precludes the possibility of any structural trap for hydrocarbon accumulation. However, detailed structural analysis shows that the structure in large scale is actually a synclinorium. The second order folds are also large at many places with wavelengths of some of these folds are as much as several km long. Some of these anticlines, smaller in dimension though, are potential structural traps. In the southern sedimentary belt (SHZ/LHZ), the large number thrusts and associated fault-related folding have given rise to extremely complex structural geometry. In this belt also possible structural traps are rather small. It is imperative that detailed structural modeling based on sound kinematic criteria needs to be carried out before any expensive exploration strategy can be adopted.
Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery