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.