Geological studies on the COST No. B-2 well, U. S. Mid-Atlantic outer continental shelf area

Abstract

The COST No. B-2 well is the first deep stratigraphic test to be drilled on the United States Atlantic Outer Continental Shelf (AOCS) area. The well was drilled on the eastern flank of the Baltimore Canyon trough to a total depth of 16,043 feet; it penetrated a section composed almost entirely of sand and shale with subordinate amounts of limestone, coal, and lignite. Biostratigraphic studies have shown that the uppermost 5,000 feet is of Tertiary and Quaternary age and was deposited in nonmarine to deep marine environments. The Upper Cretaceous section is about 3,000 feet thick and is of dominantly shallow marine origin. The basal 8,000 feet of sediment has been tentatively determined to be entirely of Early Cretaceous age, the basal sediments being dated as Berriasian. This Lower Cretaceous section is primarily nonmarine to very shallow marine in origin. Examination of cores, well cuttings, and electric logs shows that thick potential reservoir sands are found through much of the section. However, porosity and permeability decrease strikingly in the deeper parts of the Lower Cretaceous section as a result of compaction and cementation. Most of the sands are quite feldspathic, and progressive decomposition of feldspar stimulates authigenic clay and silica formation. Studies of color alteration of visible organic matter, organic geochemistry, and vitrinite reflectance show that although many units have high organic-carbon contents, moderately low geothermal gradients may have retarded thermal maturation. This, in conjunction with the scarcity of marine-derived organic matter in the lower part of the section, suggests a relatively low potential for the generation of liquid hydrocarbons. However, the overall combination of source beds, reservoirs, seals, structures, and thermal gradients may be favorable for the generation and entrapment of natural gas. Furthermore, the presence of reservoir rocks, seals, and trapping structures may indicate a significant potential for entrapment of either natural gas or petroleum that was generated deeper in the basin and then migrated either laterally or vertically.