Abstract: High Resolution Sequence Stratigraphy and Reservoir Characterization of L-III, Bombay High, India

Uppal, S. and James Peters - Keshava Deva Malaviya Institute of Petroleum Exploration, Oil & Natural Gas Corporation Ltd.

Bombay High field is India's largest hydrocarbon producer and contributes to more than 50% production in India. It lies about 160 Km. off the coast of Mumbai in western offshore. Production is mainly confined to carbonate reservoir of Early Miocene times. Drilling activities were started in 1975 and since then a number of reservoirs yielded hydrocarbons. L-III reservoir is the most prolific producer among these reservoirs. Over the years this reservoir has been extensively explored and developed. Sedimentological studies carried out so far in this reservoir indicate heterogeneity and complexity in the field which hampers the production performance and water injection schemes. In the present work all the data generated earlier has been re-examined form the viewpoint of high-resolution sequence stratigraphy to redefine the various flow units. Such an interpretation also brings out the controls on porosity-permeability and their relationship to different microfacies and the effect of diagenesis on these reservoirs.

L-III reservoir is Burdigalian in age and has been deposited in inner-neretic environment. This reservoir is subdivided into twelve sublayers which are separated by marker beds. These marker beds are generally shaly layers but at placed graded into argillaceous limestone. These may be imperfect, impervious layers between the layered reservoirs. Since L-III top is an unconformity, all the stratigraphic correlations are done on the basis of a shale marker within L-III, earlier designated as "P" shale. Integration and interpretation of available data of L- III reservoir leads to the conclusion that reservoir parameters such as porosity, permeability are independent of microfacies and dependent of diagenetic process which took place in the vadose zone and close to it. Secondary porosity in the form of soln. vugs, molds & channels in tandem with altered primary porosity are two main contributors to porosity development in L-III reservoir (Fig.1).

To understand porosity-permeability and to redefine flow units, high-resolution sequence stratigraphy has been carried out. Eight such high-resolution parasequences have been identified in L-III reservoir irrespective of earlier defined sublayers & marker beds (Fig.2) and same are correlated with other wells across the field. The limestone of different parasequence are considered to be deposited during high stand system tracts and overlying high gamma shale have been preserved during transgressive system tract/or period of maximum flood surface in areas which are subaerially exposed.

During the initial phase of HST, when water depths are more, a lagoonal mudstone bioclastic facies develops and as the sedimentation continued there is less accommodation available and comparatively high energy facies develop as Algal-Foram/Wackestone-Packestone facies. Subsequently, these facies prograde towards the basin. The subaerial exposures are attributed to low stand system tract and diagenetic changes related to it. Large solution vugs and cavities are related to such surfaces and are the result of dissolution that takes place in vadose zone. Development of better porosity-permeability is expected to be related with such surfaces irrespective of microfacies. Identification of a different parasequence could help in redefining the different flow units in L-III reservoir and their relationship with porosity and permeability.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil