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AAPG Bulletin


AAPG Bulletin, V. 86, No. 12 (December 2002),P. 2023-2045.

Copyright ©2002. The American Association of Petroleum Geologists. All rights reserved.

Application of a ramp/flat-fault model to interpretation of the Naga thrust and possible implications for petroleum exploration along the Naga thrust front

W. Norman Kent,1 Robert G. Hickman,2 Udayan Dasgupta3

1Kent GeoScience Associates, P.O. Box 1082, Richmond, Texas, 77406; email: [email protected]
2Structural Solutions, 1330 Sugar Creek Boulevard, Sugar Land, Texas, 77478; email: [email protected]
3Hindustan Oil Exploration Co., LTD, HOEC House, Tandalja Road, Vadodara, India 390 020; email: [email protected]


W. Norman Kent received a B.S. degree in geology from the University of Arizona (1970) and an M.S. degree in geology from Northern Arizona University (1975). He spent 25 years with Unocal Corporation as an exploration geologist (1975-2000). His background includes field mapping, well site geology, lease sale acreage evaluation, prospect generation, integrated regional studies, and project management. He participated in and led exploration projects in North America, North Africa, the Middle East, India, and China. He now operates a consulting firm specializing in problems related to exploration in areas of limited data and complex structure, with active projects in India, Alaska, and eastern North America.

Robert Hickman received a B.S. degree in geology from Stanford University and an M.S. degree and Ph.D. in geology from the University of Wisconsin. He spent 25 years with Unocal Corporation as a structural geologist and coordinator of structural geology. His areas of expertise include analysis of compressional, extensional, and salt regimes and integrated regional studies. He is currently a consultant specializing in structural analysis.

Udayan Dasgupta is currently general manager for Hindustan Oil Exploration Company Limited (HOEC) in Baroda, India. He received an M.Sc. degree in geology from Calcutta University, India, in 1969. Subsequently, he migrated to Canada and obtained a Ph.D. from the University of Toronto in 1978 for his research on fracturing in Mississippian carbonate reservoirs of Alberta. During this period, he also worked for Cominco Limited in exploring for carbonate-hosted base metals. He returned to India and joined Oil India Limited (OIL), where he worked in various capacities from exploration and development to strategic planning. During his early years with OIL, he worked in northeastern India, including the area covered by this article. He joined HOEC in 1994.


Hindustan Oil Exploration Company and Unocal Corporation conducted a significant part of the data collection and analysis that led to this article during a joint investigation of Block AAP-ON-94/1. We wish to thank both companies for permission and aid in publication of this article. We also thank our colleagues at Unocal and Hindustan Oil Exploration Company for their help in preparing this article. We gratefully acknowledge the support of Oil India Limited, who allowed us to use the maps and data without which the research and documentation of this article would have been impossible. Geo-Logic Systems, LLC provided valuable aid in allowing us to use LithoTech structural modeling software for testing the viability of our interpretations. For this generosity, we are particularly appreciative. We extend special thanks to Charles Stewart, David Courtis, Steven Boyer, Shankar Mitra, Robert Milici, and W. A. Bally for their helpful critiques of this article.


The Assam-Arakan thrust belt extends along the India-Myanmar border, from the Chinese border on the north to the Bay of Bengal on the south. Tertiary nonmarine sediments dominate the stratigraphy within the frontal zone of this fold and thrust belt. Thrust-fault flats occur in the upper Barail Group coaly interval, and ramps are localized by preexistent normal faults and stratigraphic discontinuities. As a result, the frontal zone of the thrust belt is characterized by multiscale imbricate structures. The Jaipur anticline occurs at the foreland edge of the Naga thrust imbricate zone, at the northeastern end of the Assam valley. Application of flat/ramp geometric models, with a limited data set from the Jaipur anticline, allows creation of geometrically viable models for interpreting the general structure of the Jaipur anticline and for developing hydrocarbon exploration leads. The results of this method indicate that (1) the proven productive foreland trend extends several kilometers beneath the Naga thrust, and (2) zones of high dip in the thrust belt mark the location of prospect leads probably related to local thrust imbrication. The method provides a testable process of identifying prospective areas; therefore, it can minimize exploration expense and optimize exploration planning.

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