MCRAE, C. D., J. O. HASTINGS, G. A. STEFFENS, J. W. HJERPE, and R. M. STOVALL, Shell Western E&P Inc., Houston, TX; and R. B. LENNON, Houston, TX

ABSTRACT: Role of 3-D Seismic Surveys in Rejuvenating Exploration and Production Prospects in Mature Areas, South Texas, United States

Within the past five years, Shell has acquired six onshore fine-grid three-dimensional (3-D) seismic surveys covering 220 sq mi in the Oligocene Vicksburg gas-producing trend of south Texas, United States. These Vicksburg fields are associated with rollover anticlines, which developed as a result of lateral translation along a decollement surface. Gas production is from deep, tight, geopressured sandstones. Individual reservoirs are cut by numerous faults, which further complicate the structural setting, making reservoir development and management of these Vicksburg fields a challenge.

The primary objectives of the surveys were first, to optimize development well locations in existing fields, and second, to expand Shell's knowledge of the structural evolution and trapping mechanisms in identifying prospective exploratory wildcat areas.

The 3-D surveys were acquired using Vibroseis, with multiplicity ranging from 24 to 45 fold and bin sizes of 75 to 100 ft.

These large surveys have rejuvenated Shell's onshore exploration effort and resulted in the discovery of one significant new field to date. The success at Cadre field has confirmed the utility of 3-D surveys for exploratory drilling, and additional surveys are planned in the future.

To date, 37 wells have been drilled in the 3-D surveyed fields with a success ratio of 92%. The gas production rate in these fields has more than doubled, from 100 to over 200 MMCF/day.

Despite the maturity of the trend, the area has continued to provide economic opportunities for Shell via the application of 3-D seismic and other enhanced technologies.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.