Stratigraphy, Sedimentology, and Reservoir Characteristics of the Lower Member of the Yacimiento El Trébol Formation

Abstract

Exploration in the offshore Golfo San Jorge Basin requires an understanding of the regional trends of the Lower Yacimiento El Trébol Fm. Sandstones. These reservoir rocks yield the best oil shows in the offshore and they are among the most prospective and prolific onshore reservoir on the Golfo San Jorge Basin's north flank. Detailed correlation enabled identification of three lower order sequences in this stratigraphic interval. These sequences are recognizable on seismic sections and well logs. The Lower Yacimiento El Trébol interval was regionally correlated from the offshore wells to the producing intervals at Escalante and El Trébol fields. No major truncations or stratigraphic wedges were identified on seismic lines or well logs, although the individual sequences that comprise this package can thin against some persistent structural highs. In addition, these sequences were correlated in 144 wells all around the north flank of the basin; sand thickness was measured from the SP curve in these wells. Net sand and sand percent maps were created. These maps demonstrate that lower Yacimiento El Trébol sandstones are concentrated in sand fairways in the Comodoro Rivadavia area and in the area south of the Marta wells. Conversely, there are many parts of the basin where Lower Yacimiento El Trébol Formation is represented by silts and shales (west and south of Comodoro Rivadavia and between Comodoro Rivadavia and Marta wells). Sand rich fairways, represented by gross thickness isopachs, map as trends that are perpendicular to the basin edges. Lower Yacimiento El Trébol production is found in these thick sand areas, while in thin sand areas, production comes from the underlying Comodoro Rivadavia Fm. Introduction The sandstones at the Lower Member of the Yacimiento El Trébol Fm. have produced over 20 MMm3 of oil as one of the main reservoirs at northern flank of San Jorge Basin. In this paper we present a synthesis of the main characteristics of these reservoirs. The Upper Cretaceous Lower Member of the Yacimiento El Trébol Fm. is defined by two regional surfaces; the lower one is a regional unconformity at the top of Comodoro Rivadavia Fm. The upper limit is a regional shale that records a basin wide flooding event. Based on log trends and markers of regular continuity, three sequences have been recognized in this unit. These sequences begin with sharp based sand packages with Christmas tree pattern and evolve upwards to shale intervals with some isolated sandstones. The stratigraphic maps of these sequences show sand concentrations with a distributary pattern 40 km wide centered at the west of the Campamento Central field. These sands pass laterally to shales to the west, east and south. A new sand concentration occurs at the location of Marta and Kaiken wells at the offshore of the basin. Sand thickness variograms show the greater continuity in NNW-SSE direction and indicate provenance from the north. Cores at the Marta wells show sandstone packages arranged in upper fining trends and interpreted as high suspension to traction fluvial channels followed by highly burrowed silt packages. These silts become dominant at the top of the interval. Cores at El Trébol field show up fining sandstone packages interpreted as fluvial meandering channels.