Multicyclic Depositional History Of Middle Eocene-Early Oligocene Carbonate Sequence Of Panna-Bassein-Heera Block, Western Offshore India: Implications for Exploration and Production Strategy

By

 Narendra Kumar Verma¹, D.K. Pande¹

(1) Oil and Natural Gas Corporation Ltd, Dehradun, India

 Middle Eocene to early Oligocene platform carbonates (>700m) of Bassein and Mukta Formations harboring number of discovered oil and gas fields around Bombay High is a complex carbonate sequence with cyclic depositional and diagenetic imprints and vertico-lateral thickness/facies variations. Discovered hydrocarbons in the uppermost porous layers of carbonate sequence lead to hitherto accepted model of single phase Middle/Late Eocene unconformity related reservoir development. However, number of hydrodynamic fluid anomalies encountered necessitate alternative solution. Chrono-stratigraphically constrained log correlation across platform area has revealed the heterochronous character of reservoirs in different structures. Eight distinctive stratigraphic units are identified depicting the transgressive- regressive cyclic sedimentation during Middle Eocene-Early Oligocene. Four hydrocarbon bearing units within Middle Eocene are identified as Neelam pay (Youngest), Bassein Pay, Heera Pay, and B-lower Pay (Oldest). Additionally charged middle Units form Mukta Pay. It implies existence of multiple phases of hiatus-diagenesis related porosity generation and stratigraphically controlled hydrodynamic fluid draining systems, which better explains fluid distribution.

It has significant implications for hydrocarbon exploration strategy. An interplay of porosity generation vis-à-vis structural disposition of respective layers seems to govern the hydrocarbon accumulation. Three set of faults genetically related to rifting and rotational shearing further influence the fluid migration and accumulation. Lateral extent of discovered pay units may be tracked down in structurally favorable areas to locate hitherto untested oil and gas pools.

Spatial traceability of identified units on reconstructed seismic sections and chronostratigraphic constraining of carbonate depositional cycles by biostratigraphic information and eustatic sea level curves establishes their stratigraphic validity.