Geochemistry in an Integrated Study of Reservoir Compartmentalization at Ewing Bank 873, Offshore Gulf of Mexico

Summary Ewing Bank 873 is an offshore Gulf of Mexico field discovered in 1991 in 775 ft of water. The discovery well was drilled on a seismic amplitude anomaly on the flank of a salt withdrawal minibasin. Field development began in 1994, and in mid-1998 daily production from the Bulminella 1 reservoir averaged 40,000 BOPD and 32×106 ft3/D of gas. The Bul 1 reservoir in this combination structural-stratigraphic trap consists of six stacked and overlapping Pliocene turbidite sand lobes. In turn, integration of seismic, well log, geochemical and pressure data indicates these six turbidite lobes comprise three compartments. All of the various data types give constraints on different aspects of compartmentalization, but at the stratigraphically complex Ewing Bank 873 field, geochemical analyses provided key information unavailable through any other means. These geochemical analyses were performed as individual wells in the field went on production and immediately provided information regarding fluid communication and reservoir connectivity that was missing from earlier interpretations based solely on seismic and log data. Early recognition of three reservoir compartments using geochemical data also helped constrain preliminary stratigraphic interpretations and provided initial input for flow units and reservoir simulation models. The geochemical information further provided advance notice of economically significant oil quality variations in the three compartments. These fluid variabilities were later substantiated by pressure/volume/temperature analyses and include notable differences in oil gravity, weight percent sulfur, viscosity and solution gas. Integrating all available data shows there are three compartments at Ewing Bank 873 and each compartment comprises different turbidite sand lobes and exhibits its own characteristic pressure regime and fluid properties. The early indications of both compartmentalization and variation in fluid properties by the geochemical analyses contributed significantly to improved field recovery and economics by allowing fewer and better placed wells to be drilled. Introduction Development of deepwater Gulf of Mexico fields is an expensive undertaking that involves considerable analysis and evaluation of both engineering and geologic data. In this context, failure to recognize reservoir compartmentalization can add significantly to field development capital and result in less than optimum reservoir management. For all of these reasons, early recognition of reservoir compartmentalization is advantageous. Traditionally, pressure, seismic and log data have been among the primary tools used to identify compartmentalization. Each of these techniques provides information on a different aspect of reservoir compartmentalization, and the various methodologies work best when integrated. For example, at Ewing Bank 873 the pre-drill seismic data provide the gross geometry and lateral distribution of the different turbidite sand packages,1 but the seismic cannot resolve individual sand lobes. Well logs do show individual sand lobes, but in this instance, where turbidite lobes are characterized by compensation-style bedding, correlation of sands is difficult. Pressure data also yield valuable information on reservoir compartmentalization, but long term build-up tests can be expensive to obtain and involve shutting in production. In addition to these techniques, another tool for identifying reservoir compartmentalization that complements the other analyses by providing a direct indication of reservoir fluid continuity is geochemistry. Over the past 10 years, a number of case studies2–10 have clearly demonstrated the value of geochemistry in recognizing reservoir compartmentalization. In particular, geochemical analysis of oils by gas chromatography is an inexpensive technique requiring minimal turnaround time that provides information on compartmentalization early in the history of a reservoir. In turn, early recognition of compartmentalization can help in the placement of development wells and optimize new field development choices. Beyond early recognition of compartmentalization, geochemical analysis at Ewing Bank 873 also helped constrain reservoir simulations, assisted in correlation of reservoir sand units and aided in understanding oil quality variations within the reservoir. This case history further demonstrates how geochemical analyses provide a unique opportunity to characterize a reservoir from the perspective of the actual reservoir fluids rather than from the perspective of the "container" holding those fluids. However, it is only by integrating all of the reservoir data—geochemical, seismic, log, pressure and stratigraphic—that reservoir management can be optimized. Geologic Setting and Field Description Ewing Bank 873 is a deepwater (775 ft) Gulf of Mexico field that is almost 200 miles due south of New Orleans (Fig. 1) and lies along the flexure trend between the current shelf and continental slope. This field was discovered in 1991 by drilling a seismic amplitude anomaly on the flank of a salt withdrawal minibasin (Fig. 2). The reservoir is a series of six stacked and overlapping Pliocene Bulminella 1 (3.8 Ma) deepwater turbidites. The trap at Ewing Bank 873 is a combination stratigraphic/structural trap formed by updip pinch out of the turbidite sands and by faults on the eastern and western margins of the field. Updip sand pinch out is in turn controlled by an underlying salt diapir.