--> ABSTRACT: Old Wells, New Tools; Troporo Field, A Case Study, by Jay Timothy Altum, Gregory G. Chapel, Mark A. Cocker; #91020 (1995).

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Old Wells, New Tools; Troporo Field, A Case Study

Jay Timothy Altum, Gregory G. Chapel, Mark A. Cocker

A geological and petrophysical study of the Devonian tripolitic chert was conducted to confirm previous estimates of original oil-in-place (OOIP). The work included visual inspection of core to characterize reservoir lithofacies, correlation and subdivision of the productive and potentially productive pays into five main zones, determination of rock properties from core-calibrated analysis of log data, and volumetric mapping by zone leading to the determination of OOIP.

The base of the tripolitic chert dips to the northwest across the Troporo Devonian Unit at approximately 4° and is truncated at its updip limit to the east by subcrop against Permo-Penn unconformity. The Devonian reservoir interval was deposited in an open marine to shelf environment and is characterized by three lithofacies, only one of which is of reservoir quality, and readily identified from log response. Northeast to southwest trending lobes of porosity range in thickness from two feet to more than forty feet with air-permeabilities ranging between .7 and 2.5 md and log porosities between 13 and 19%.

A revised volumetric OOIP is estimated to be 27 percent larger than earlier estimates and equivalent to only 16 percent recovery efficiency under waterflood. Such a low rate of recovery suggests a potential bypassed oil target. A program to confirm the larger volume was conducted to model the distribution of reservoir rock and define the erosional edge and fault patterns within the tripolitic chert.

3-D seismic data was interpreted to determine the internal geometry of the tripolitic chert intervals, image the updip truncation to the east by subcrop against the Permo-Penn unconformity and redefine fault configuration. Development of an amplitude/porosity correlation was instrumental in identifying potential areas for additional reserves.

The seismic, geology and petrophysical parameters were integrated in a 3-D visualization and attribute analysis program. A 3-D volume was generated, constrained by the seismic, and petrophysical parameters were modeled field wide. Solid geobodies were constructed using seismic amplitude and petrophysical criteria determined earlier to define reservoir rock. Connectivity of geobodies, horizontally and vertically, was extrapolated across the field. A computed volume within modeled geobodies confirmed the OOIP. The integration of geological, geophysical and petrophysical attributes into a 3-D analysis and visualization program combined with an integrated reservoir engineering study could result in optimized infill well locations and additional field wide development.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995