MYERS, KEITH, BP Exploration, Middlesex, England

ABSTRACT: The Sequence Stratigraphy of Marine Carbonate Source Rocks

Many of the world's most prolific source-rocks are developed within marine carbonate depositional systems. Marine carbonate source rocks are significant also in that they generate oils with a distinct character (high S, low Pr/Ph <1.5) at significantly lower temperatures than other classes of source rock.

The depositional controls on marine carbonate source rocks are very similar to those for marine siliciclastic source rocks. The development of oxygen deficient/depleted (ODD) bottom water conditions is critical, either as a result of restricted circulation or high surface productivity. Terrigenous organic matter supply generally tends to be less important given the arid climates in which many carbonate depositional systems develop.

Major carbonate source rocks can be placed into a coherent sequence stratigraphic framework and classified into four main genetic types based on depositional geometry and controlling mechanism. Carbonate depositional systems differ from clastic systems in that they can create the physiogeographic restriction required for source rock deposition by 'upbuilding' in response to rapid relative sea level rise.

The inter carbonate buildup source rock develops predominantly in transgressive systems tracts in areas of restricted circulation between carbonate buildups. The intra shelf basin source rock develops predominantly in transgressive systems tracts when an isostatically sagged platform interior is drowned. Geometrically these two source rock types can look similar and differ mainly in the driving mechanism. Both are characteristic of high carbonate productivity systems. Unrestricted carbonate margin source rocks develop in transgressive systems tracts on lower carbonate productivity margins during period of upwelling and/or oceanic anoxia. Deep 'ocean' basin carbonate source rocks develop in bathyl water depths in long-lived tectonically silled carbonate fringed anoxic basins. Prolific ource rocks have been deposited in all of these settings. Note that a working carbonate source system in a basin may contain more than one of the above source rock types.

Each of these source rock types will be discussed in turn addressing their sequence stratigraphic context, critical factors for their prediction and their distribution through time.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.