Dilip Mukhopadhyay

Dilip K. Mukhopadhyay

Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee 247 667, Uttarakhand
e-mail: dilipfes@gmail.com

In the last few decades, the techniques of cross-section balancing have been applied to many fold-thrust-belts (FTBs)/collision zones in many parts of the world with spectacular results in terms of both successful exploration projects and academic research. The Himalaya and Assam-Arakan (AA) belt constitute one of largest continent-continent collision zones in the world leading to the formation of a thousands of km long FTB, which in part is hydrocarbon bearing and also seismically very active. A majority of balanced cross sections in the Himalayan FTB is by foreign researches or their collaborators. With a few notable examples, there is a general lack of enthusiasm for this technique among Indian geoscientists. It appears that this lack of enthusiasm has spawned a few myths and misconceptions about this technique, including (but not limited to):

Myth 1. This is primarily a software-based research, far removed from reality. This is false. Even to this day no structural modelling software is available that will automatically generate balanced cross section.

Myth 2. All balanced cross sections look similar. Once again this is false. It has been shown that within a single salient or recess the structural geometry and evolution can vary significantly.

Myth 3. Balanced cross sections cannot be constructed without subsurface data. This may be due to the fact that this technique originated in the industry where seismic and logs are usually available. However, the beauty of the technique is that it can be implemented with incomplete data set.

Myth 4. Section balancing is invalid because it does not take into consideration stress and strain. Most of the structural models used in section balancing are kinematic models. Kinematics is a valid branch in fluid dynamics leading to a mathematical description of deformation. In any case, strain, if it can be quantified, can be considered while restoration is carried out.

By definition, a balanced cross section is a deformed-state cross section that is both admissible and viable. It is like any other deformed-state cross section with an important distinction: the section has been tested for geometric admissibility and kinematic viability. Balancing a cross section is a tedious and time-consuming process because at every stage of section construction admissibility and kinematic validity need to be maintained. Schematic cross sections, on the other hand, take a fraction of time to draw because they are based more on imagination and intuition. In recent years, the availability of structural modelling softwares has made the “tedious” part of the balancing a little less tedious!

Section balancing is an extremely important technique regularly used by petroleum industry. This is because a great majority of oil/gas traps are structural traps, and consequently, a reliable subsurface structural interpretation is of paramount importance for prudent fiscal policy.

In academia, three major advantages can be envisioned at least in sedimentary terrains: (1) quantifiable and internally consistent sub-surface structural interpretation, (2) deduction of structural evolution through kinematically valid step-wise restoration, and (3) estimation of orogenic shortening.

The number of balanced structural cross sections available in the literature in the thousands of km long Indian collision zone is awfully inadequate. In the last two decades or so, a few sections have been published mostly in the Himalayan foreland FTB. AA belt largely remain unexplored.