Lithostratigraphy and Hydrocarbon Prospects of the Olpad Formation of Paleocene-Early Eocene Age in Ahmedabad-Mehsana Tectonic Block, Cambay Basin

The Olpad Formation of Paleocene-Early Eocene age represents the oldest Tertiary clastic deposits. in the Mehsana area, North Cambay Basin. This unit is characterised by its heterogeneous lithologic association of volcanic conglomerate, volcanic wacke, trapwash, siltstone and shale, deposited by alluvial fan systems along fringes of palaeohighs and lacustrine to brakish water condition in the existing palaeolows. Recent exploratory drilling proved the occurrence of hydrocarbons in this formation. This significant find has prompted the necessity of identifying potential hydrocarbon reservoirs. The formation has been subdivided into three lithostratigraphic units (Unit I - III in descending order) based on sedimentary facies and well log characteristics. The reservoir characteristics of the sediments of Olpad Formation has been worked out based on sedimentological, SEM and XRD studies.

Lithostratigraphic correlation, structural configuration and thickness variation have been discussed. The study reveals that the distribution of different lithostratigraphic unit is controlled by tectono-sedimentation factors and the volcanic wacke lithofacies of Unit-I and III constitute the reservoir rock in the Olpad Fonnation. The study led to the identification of Umiwa. Khambel and Gamanpura as prospective areas for future exploration for structural-cum-stratigraphic traps.