East Breaks 160 Field On The Offshore Texas Shelf Edge: A Model For Deepwater Exploration And Development

ABSTRACT East Breaks Block 160 Field, approximately 100 miles south of Galveston, Texas in excess of 600 feet of water, was the earliest attempt of commercial hydrocarbon development beyond the offshore Texas shelf edge. East Breaks Block 160 Field consists of two blocks, East Breaks 160 and 161, leased by UNOCAL, Mobil and Amoco in 1974. The original structural interpretation covering this field showed the trapping mechanism to be four-way dip closures on the downthrown side of a down-to-the-basin growth fault. The original reservoirs were assumed to be laterally continuous Pleistocene sandstones. Between the 1974 lease acquisition and 1980 platform Cerveza installation, UNOCAL and partners drilled six exploratory wells on the blocks and continually acquired newer and more sophisticated seismic surveys. The most recent seismic acquisition on the field was a 3-D seismic survey which resulted in reducing the number of development wells from 35, pre-survey to an ultimate 23 wells. The resulting development drilling and 3-D survey showed the structural traps in the field to be primarily three-way downthrown fault closures. This additional data also caused a revision in the stratigraphic model to that of a laterally discontinuous series of slope-deposited sand bodies. INTRODUCTION The objective of this paper is to present the chronological events while illustrating the results of the evolution of geological and geophysical techniques, beginning with initial exploration to production start-up of the East Breaks 160 Field. East Breaks 160 Field was the first development project beyond the offshore Texas shelf edge. The East Breaks Blocks 157-161 lie 100 miles south of Galveston, Texas, in excess of 600 feet of water on the upper Continental slope (Figure 1). These five blocks were originally leased by Union Oil Company of California (now UNOCAL) and their partners, Mobil and Amoco, in June of 1974. EXPLORATION The original structural interpretation on this prospect, made prior to the 1974 lease sale (Figure 2) at the middle Pleistocene MP-3 horizon, was drawn as downthrown four-way dip closures on a down-to-the-basin major growth fault with associated synthetic faulting. The objective stratigraphy was assumed to be Pleistocene sands and shales. The nearest available well control was approximately 40 miles to the east-northeast in High Island Block A-572. The stratigraphic model assumed laterally continuous sand bodies. These structural and stratigraphic models were developed using updip continental shelf analogies. The seismic data used for this structural interpretation was a loose 1/2 mile by 1/2 mile grid of unmigrated, low fold data. Seismic Line A (Figure 3) demonstrates the anticlinal rollover into the down-to-the-basin faults. Line A also shows high amplitude reflectors near the MP-3 horizon. These crestal reflections were considered to be indicative of hydrocarbons. Following the purchase of the blocks in June 1974, a joint exploratory drilling program was undertaken by the partnership. The OCS-G-2648 #1 well East Breaks Block 161 drilled to a total depth of 9700 feet and was one of the earliest wells drilled on this feature.