TUCKER, Maurice E., Geological Sciences, Durham University, Durham, England

ABSTRACT: Carbonate Sequence Stratigraphy, Diagenesis, and Porosity Prediction

Considering carbonate rocks in the context of changes of relative sea level and accommodation space enables a degree of prediction of sediment body geometry and stacking patterns, and of the course of early diagenesis and evolution of porosity.

During a major sea level fall and in a humid climate, the sediments of the previous highstand systems tracts (HST) and transgressive systems tracts (TST) are subjected to meteoric leaching and cementation, and karstification from the sequence boundary. Both porosity occlusion and enhancement may occur. In an arid climate, reflux dolomitization is likely to be important. TST facies are typified by marine cementation, especially if the carbonate platform is in a windward orientation, followed by burial in marine pore fluids where no significant diagenetic reactions take place until compaction begins or meteoric flushing occurs. TST facies have major reservoir potential, commonly retaining significant primary porosity into the deep burial realm. If dolomitization by circulating seawater s an important process, then it is most likely to occur during the TST, when the relative sea level rise pushes marine groundwaters through the sediments. Very porous rocks can be produced in this way if there is concominant aragonite dissolution. During the HST, sediments may be subjected to marine cementation, but this would soon be followed by meteoric diagenesis in a humid climate or by evaporative dolomitization if the climate is arid.

Many carbonate platforms consist of numerous parasequences, and their diagenesis depends on their position within the sequence. Those parasequences deposited during the third-order sea level fall generally show the effects of surface-related diagenesis (supratidal dolomitization or karstification) to a much greater degree than those deposited during the third-order sea level rise. Relative sea level changes have varied through time, and these have had a strong influence on the nature of sequences and parasequences, as well as on their diagenesis.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.