Petroleum Generation and Migration in Lower Jurassic Sequences, Hartford Basin, Connecticut and Massachusetts

Lisa M. Pratt, Robert C. Burruss, Paul E. Olsen

The Hartford basin is one of a series of elongate fault-bounded basins in eastern North America containing Triassic and Jurassic sedimentary rocks. The Lower Jurassic Shuttle Meadow and East Berlin Formations contain 8 to 10 deep-water lacustrine cycles (1-1.5 m thick) composed of gray silty mudstone and black laminated shale.

Temperatures of maximum pyrolytic yield (T_max) and hydrogen indices (HI) of 66 samples of shale and siltstone (TOC = 0.9-3.8 wt. %) from the Shuttle Meadow and East Berlin Formations indicate sharply decreasing thermal maturity from north to south. In Massachusetts and northern Connecticut, these units have T_max > 460°C and HI < 50 mg/g, indicating potential for gas but overly mature for oil. In central and southern Connecticut, T_max is 445°-425°C and HI is 150 to 440 mg/g, indicating moderate to good oil potential. The less mature shales have solvent extract yields of 1,500-3,100 ppm. N-alkanes are smoothly distributed with a maximum between n-C₁₇ and n-C₁₉, typical of good petroleum source rocks.

Bitumen-coated calcite spar in jointed Holyoke Basalt, and silica and carbonate cements in a concretion from the East Berlin Formation contain aqueous liquid and vapor inclusions and abundant petroleum inclusions. The petroleum inclusions variously contain fluorescent hydrocarbon liquid, nonfluorescent brown solids (bitumen), vapor, and, rarely, aqueous liquid. Preliminary measurement of homogenization temperatures are 75°-115°C. The regional setting, presence of bitumen-coated fractures, and occurrence of petroleum liquids in fluid inclusions with temperatures in the range of the oil window demonstrate generation and migration of oil during burial maturation in Hartford basin.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.