Abstract

Carbonate Sequence stratigraphy is at variance to the standard sequence stratigraphic model, because the standard sequence stratigraphic model was originally developed for marine clastic depositional systems. The difference is owing to the fact that to a large extent carbonates are born within the depositional environment and forms certain geometries like rimmed platforms, reefs, atolls etc. unlike the clastics which are transported and deposited. However, the fundamental features of the model is shared by all the depositional systems irrespective of their composition, scale and physiography, therefore it is applicable to carbonates as well, with little modification in the approach and by understanding the inherent characters of the carbonate. High frequency sequence stratigraphic study carried out in the NBP (D1) area of Mumbai offshore basin to address the spatio-temporal porosity variation and hydrocarbon distribution of carbonate reservoirs which are below seismic resolution. Detailed workflow for high frequency sequence stratigraphic study has been formulated which can be readily adopted in the industry practices using seismic, well log, core, cuttings, biostratigraphic, SCAL and pressure/production data. Higher order sequence stratigraphic framework of the area was established first in a semi-regional context using the seismic data. On the basis of seismic reflection pattern 1st and 2nd Order sequences were identified which were tied to the well log data and correlated across the area. The entire sedimentary column from Deccan trap top to sea bed was divided into two 1st Order Sequences such as, (1) Paleocene to Early Eocene, Syn-rift sequence and (2)Eocene to Recent, Passive margin sequence, Passive margin sequence consists of two 2nd Order sequences viz. (i) Middle to Late Eocene and (ii) Oligo-Miocene. High frequency sequence stratigraphic study was carried out on the hydrocarbon bearing Oligo-Miocene 2nd Order sequence only, through demarcation of 3rd, 4th and 5th Order sequence. For the higher order seismic scale sequences depositional sequence approach was adopted whereas for lower order sub-seismic scale sequences genetic sequence approach was applied. Facies prediction in carbonate is non-linear; at the same time carbonate has the intrinsic character of chemically highly reactive and gets lithified within short span of time and very less susceptible to mechanical erosion on subaerial exposure which influences the sediment compartmentalization in a different way than that of siliciclastic systems across the basin during the various stages of base level cycle, such as lowstand shedding of siliciclastics vs. highstand shedding of carbonates.