Abstract

A 3-D geologic model of the Hanifa reservoir was constructed using sequence stratigraphic principles and facies to control the distribution of porosity and permeability. The reservoir had been previously interpreted as having "layer cake" stratigraphy based on correlation of similar lithologies and similar appearing porosity inflections. The new geologic model incorporates a field-wide gamma ray correlation over 55 km from the non-reservoir rocks to the main producing area of the field. The gamma ray correlation produced a previously unrecognized reservoir geometry consisting of a high stand systems tract, a shelf (ramp) margin wedge, and a transgressive systems tract. This model represents our first 3-D geologic model based on sequence stratigraphy. This geologic model has been used successfully in reservoir simulation and field operations.

The reservoir consists of skeletal sands and stromatoporoid boundstone complexes to the north that grade to non-reservoir mudstones to the south. The reservoir was divided into 45 geologic layers. Core descriptions led to mapping facies distribution within each geologic layer. A 3-D lithofacies model was constructed and used as a template to calculate and assemble porosity and permeability models. A water-saturation model was also constructed based on facies specific J-functions. The geologic horizons were then grouped into 20 flow layers as the geologic front end into reservoir simulation.

The resulting geologic model has improved the Hanifa reservoir simulation by defining previously unrecognized reservoir geometries and providing the detailed resolution that enabled engineers and geologists to identify and understand the geologic attributes controlling fluid movement. The flood front has moved preferentially through grainstones in the highstand systems tract by crossing layer boundaries. Fluid flow has been impeded within layers that are dominated by boundstones, wackestones or mudstones.

Introduction

Berri field is located about 100 km north of Dhahran in the Eastern Province of Saudi Arabia. The field ranks as the 22nd largest in the world (Carmalt and St. John, 1986) and is in a mature state of development. A peripheral water flood has been in effect since 1973. During the past 20 years, water encroachment within the reservoir has been poorly understood. Previous geologic and simulation models have failed to explain and predict water movement.

In late 1988, Saudi Aramco commissioned Mobil Exploration and Producing Services to conduct a full field geological and engineering description of Berri Hanifa and Hadriya reservoirs. Mobil geologists, working closely with Saudi Aramco counterparts, developed a detailed geological understanding of the sedimentology, depositional environments and diagenesis of this reservoir using all available core and petrophysical log data (McGuire, 1992).

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