Abstract: Deep-Water Carbonate Sedimentology, Diagenesis, and Petroleum Potential
Peter A. Scholle
Major oil and/or gas production now has been established in deep-water carbonate rocks of the United States, Mexico, the Middle East, and the North Sea. These reservoirs are in two main categories: pelagic deposits and transported redeposited shallow-water material.
Pelagic carbonate rocks, mainly chalk, are widespread and constitute about 75% of the carbonate sediments deposited worldwide for the past 100 m.y. Because of their initial low-magnesium calcite composition, deep-water depositional setting, and low permeability, pelagic limestones are not particularly susceptible to the freshwater alteration that normally affects shallow-water carbonate deposits. Rather, depth of burial is the critical factor in such carbonate materials, for overburden stresses set up mechanical and chemical (solution transfer) compaction that reduces porosity. Physical and chemical evidence indicates that only through early initiation of overpressuring, early oil input, or lack of burial can the high primary porosities of pelagic carbonate deposits be retained, as ex mplified by chalks from the North Sea, which are prolific oil producers. Later diagenetic fracturing can lead to smaller, but still important, reservoir potential, as in the Austin Chalk of Texas.
Transported shallow-water carbonate rocks have depositional and diagenetic patterns similar to those of platform types. These allodapic limestones are present as fans adjacent to platform margins or as layered turbidite complexes in trenches or abyssal plains. Unstable components make these sediments prone to leaching, cementation, and dolomitization. As in untransported shallow-water carbonate deposits, the forms of porosity preservation, reduction, or creation may be complex and difficult to predict. Yet major reserves of hydrocarbons can be found in such settings (e.g., the Tamabra production in Mexico). As exploration shifts from continental shelves to slope and rise areas, such reservoirs will become increasingly common and important.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC