RESERVOIR CHARACTERIZATION AND GEOMETRIES OF JURASSIC EAST BERLIN AND PORTLAND FORMATIONS, HARTFORD BASIN, CONNECTICUT, USA

The Jurassic East Berlin and Portland Formations exposed in the eastern Hartford Basin are composed of strata deposited during the thermal subsidence phase of a continental rift sequence. These strata provide important potential reservoir analogs for productive units in other rift basins, such as the northern North Sea.

Five lithofacies are recognized: 1) cross-bedded and channelized cobble conglomerate, 2) coarse-grained, cross-bedded arkose, 3) fine-grained, rippled and mud-cracked arkose, 4) red siltstone, and 5) black mudstone. Reservoir facies (conglomerate and arkose) are most common at the basin margins and in the upper portion of the basin fill where sedimentation outpaced increases in accommodation. They contain depositional characteristics that could make them excellent reservoirs, including a high net-to-gross and good connectivity.

Basin margin strata are constructed of amalgamated channels containing conglomerate and coarse arkose. The outcrops observed (up to 13 m thick) are essentially 100% reservoir, and lack vertical flow barriers. These strata are interpreted as alluvial megafan deposits composed of amalgamated braided-stream channels and bars.

The red siltstone (non-reservoir) lithofacies occurs within 2 km of the basin edge. Initially it occurs as thin (5 to 20 cm), discontinuous layers between channels. Toward the center of the basin, the layers increase in thickness and abundance. In the lower Portland Formation, some layers are continuous and have been used to subdivide the succession into accommodation-related cycles, 4-11 m in thickness. Here, the red siltstone contains green mottling and is interpreted as floodplain paleosols. Reservoir facies are thick, amalgamated sandstone channels with a net-to-gross of 90 to 95%, similar to those of the Triassic Skagerrak Formation of the northern North Sea.

The East Berlin Formation in the center of the basin is characterized by fine and coarse arkose, red siltstone, and black mudstone that represent lacustrine, braided stream, and floodplain environments. It exhibits well-developed cyclicity, 2-7 m thick. Reservoir sandstones are thin (less than 3 m) and vertically compartmentalized, and outcrops have a net-to-gross of 20 to 30%, similar to the Cormorant, Lunde, and lower part of the Statfjord Formations in the northern North Sea.