Implications of the Deformational Processes for Petroleum Exploration in the Fold-and-Thrust Belt of the Himalaya
A. R. Bhattacharya
Centre of Advanced Study in Geology, University of Lucknow, Lucknow
In petroleum exploration, structural geology undoubtedly plays a major role, especially in the Himalayan region. Success depends much not only on a clearer understanding of structural complexities of rocks but also on the various deformational processes that virtually govern and control the flow system including both solids (rocks) and fluids (water, oil, mineralizing fluids, etc.) and thus the formation, migration and localization of petroleum. In recent years, the entire fold-and-thrust belt of the Himalaya is being zoomed-in for petroleum exploration. In the paper we have highlighted the implications of the various deformational processes in the (A) Krol belt of the Lesser Himalaya that is delimited to the south by the moderately north-dipping Main Boundary Thrust (MBT) from the (B) Siwalik belt that is delimited to the south by another north-dipping tectonic plane, the Himalayan Frontal Thrust (HFT).
The most obvious effects of deformation in rocks are the development of a variety of structures which in turn help in quantitative estimation of tectonic strain. Of all the structures in the Himalayan rocks, it is only the folds that are developed practically in all the rock formations. Author’s estimation of flattening strain in folds of all the lithotectonic units of the Himalaya indicates that in general the Himalayan rocks show low to moderate strain levels ranging from 4 to 45 % across much of the mountain terrain excepting the Main Central Thrust (MCT) zone (the MCT separates the sedimentary belt of the Lesser Himalaya from the crystalline rocks of the Greater Himalaya); in the MCT zone, ductile strain progressively increases to > 90% towards the trace of the MCT from both (north and south) sides within a tract of about 15 km or so. In the Lesser Himalaya, the rocks of the Outer Sedimentary Belt (i.e. Krol belt) show much lower strain levels, up to 18 % only, than those of their northern counterpart, i.e. the Inner Sedimentary Belt, up to 40 %. This fact appears to have significant implications for petroleum exploration.
The lower Krol rocks predominantly include thinly-bedded limestone interbedded with shale bands of varying thicknesses (mm to cm scales) and commonly show small-scale duplex structures, ramp-flat geometry and minor thrust planes in the lithologic layers as well as on the limbs of folds. The formation of minor thrusts in all such cases may have caused local thickening of the competent layers, thus making the layers rather more resistant to bending and thus to folding. This situation prevented, or at least not favoured, post-buckle flattening in the Krol rocks, which thus exhibit relatively low levels of strain despite the fact that their ages and period (length of time) of exposure to the prevailing collisional stresses are the same or comparable to the rocks of the Inner Sedimentary Belt. The rocks of the Krol belt have thus remained under the influence of a regional shear stress dominantly directed to the south, as also revealed by the disposition of the Krol rocks as an up-thrust block against the north-dipping MBT. This implies that the rock masses as well as the fluid system of the Krol belt are getting pushed southwards towards the prominent anisotropic plane, i.e. the MBT which thus can be considered as a suitable candidate for petroleum exploration.
In the Siwalik rocks, development of minor folds is rare and the flattening strain is very low, up to 5 % only, and therefore accommodation of compressive stresses through folding and later superposition of strain seems to be very low to negligible as a major part of the compressive stresses were utilized in the development of complex thrust geometry (e.g. duplex, imbricate fan systems, pop-up structures, snakehead anticlines and duplexes, fault-propagation folds, antiformal stacks, overstep thrust systems, and a variety of related structures) implying that the Siwalik rocks may have, or are still undergoing, a good deal of “structural thickening” in internal domain. This may have disturbed the “critical taper” of the Siwalik wedge. In order to maintain a constant critical taper of this wedge, the mountain front thus should shift southwards, i.e. towards the foreland (author’s “Expanding Mountain Front Hypothesis”). As a result of progressive internal deformation in the Siwalik wedge, new thrusts must be forming at the mountain front and the early-formed blind thrusts are progressively becoming emergent thrusts such as the HFT. The Siwalik wedge is thus persistently getting pushed to the south. Since the thrusts greatly accommodate the tectonic stresses, the fluid system should also accommodate itself within these thrusts which thus could be the potential sites for petroleum localization.
Further, the early low-angle, layer-parallel bedding-plane thrusts are generally not believed to be associated with fluids or hydrocarbons (that might have migrated from the underlying Subathu sediments) as these thrusts are more responsible for reducing the porosity of the rocks during dewatering and lithification (as exemplified in the Nankai trough of Japan fore-arc basin). The later high-angle faults, on the other hand, are probably significant fluid conduits for the hydrocarbons that might have been trapped due to their migration from the adjoining/underlying Subathu rocks at a more advanced stage of lithification.
The Main Boundary Thrust and the high-angle faults of the Siwalik belt as well as the Himalayan Frontal Thrust could thus be the possible sites for localization of oil and gas. It is therefore hoped that the exploration programmes should also take these aspects into consideration before drawing any final conclusion.
Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery