Flow Unit Characterization and Geo-modeling of a Structurally Complex Fluvio-deltaic Reservoir using an Integrated Approach
Authors: Sharma Varun , Dasgupta Sagnik , Mahesh Arathi. L. , Sharma Sachin , Barua Indrajit , Bharali Binayananda 
Published: March 2009

The Lower Tipam sandstone reservoir of Miocene age in the Jaipur oilfield lies within a highly folded and faulted Assam-Shelf basin, in the north eastern part of India. The analysis of this mature field carries a lot of interest not only because the sands within the formation are hydrocarbon bearing but also because of the complexities associated with its evaluation. The complexities in general relate to a heterogeneous reservoir with complex mineralogy, varying water salinities across the field which makes the visualization of a conceptual geological model in the presence of a complex structure a real challenge. The objective of the study was thus to characterize the reservoir at wellbore level and conduct realistic inter-well and reservoir-scale geo-modeling for improved oil-field development by means of a comprehensive, interdisciplinary approach.

The Jaipur area is mainly characterized by a Tertiary terrigenous sedimentary sequence comprising of fluvial to deltaic deposits overlying the Precambrian metamorphic Basement. The reservoir is oil bearing without a gas cap. The clay typing and the salinities of water bearing zone have an important bearing on the hydrocarbon saturation computation.

Based on the available core data, production data and the volumetric computation at wellbore level using ELANPlus™, the field portrayed oil occurrence in a unique pattern when visualizing the ‘oil down to’ i.e. OWC, oil-shale contact (OSC) and wells with residual oil saturation. The pattern appeared unusual in a homogeneous clastic reservoir.

The OWC depth in the reservoir showed variations across the field, suggesting the presence of a fault network which also controlled the hydrocarbon entrapment. These faults were not easily identifiable from seismic interpretation; the discrete fault network of the area was analyzed by a process of automated fault identification. Though the difference in the OWC encountered in the wells could be explained based on the sealing faults barriers picked up in the area, but the pattern of the depths of OWC, OSC and distribution of wells with residual oil saturation areally suggests the presence of a stratigraphic trap within the formation. The 3D seismic volume was analyzed to understand the extent of the sand body forming the stratigraphic trap and to define its morphology. Iterative analyses of the seismic attributes suggested the possible geometry of the sand body. Using the log interpretation, an isochore was prepared as per the evolved morphology of the sand and was used for modeling the sub zones.

The rock types were modeled geo-statistically to define the spatial heterogeneity and to visualize the distinct compartments within the reservoir. Using single well predictive modeling, the flow units and the petrophysical rock types were defined and the water saturation in the 3 dimensional spaces was modeled through saturation height function for each of the reservoir rock types which were later used for HCIIP calculations.

As a result of this study the geo-cellular model of the Lower Tipam reservoir was prepared incorporating a better understanding of the spatial heterogeneity, flow unit definitions and the fault compartmentalization.