PALEOCLIMATIC CONTROLS ON SEDIMENTATION, DIAGENESIS, AND RESERVOIR QUALITY - LESSONS FROM MIOCENE CARBONATES

Miocene carbonates worldwide were deposited in a wide range of paleogeographic and tectonic settings and form important reservoirs in several regions. The reservoir quality of these carbonates appears to have been primarily controlled by the prevailing paleoclimate, which exerted a major control on both the depositional patterns and diagenetic pathways. Two end members are discussed here: (1) humid, oceanic tropical-subtropical settings, and (2) arid, land-locked temperate-subtropical settings. In humid, oceanic tropical-subtropical settings (e.g., Miocene of Southeast Asia), a warming trend paralleled by generally rising sea level allowed thick coral reefs and skeletal banks to develop, most of which are seismically resolvable. These carbonates typically represent several stages or cycles of development (largely third-order) separated by discontinuities in platform growth with episodic subaerial exposure. Development of economic reservoirs is mainly related to relative sea level falls and associated meteoric diagenesis. Trends and patterns of reservoir distribution can be predicted within a sequence stratigraphic framework. Basal transgressive carbonates are mostly tight because of their relatively fine-grained textures, intense compaction, and isolation from meteoric water influence. Best reservoir quality is commonly developed beneath subaerial unconformities in high-stand buildups where effects of meteoric water leaching and karstification are most intense. In contrast, in arid, land-locked, subtropical-temperate settings, unfavorable environmental conditions caused by periodically increased salinity and, probably, relatively low temperature restricted the full growth of carbonate buildups. As a result, relatively thin and narrow fringing coral reefs with small,. poorly developed lagoons in rhodalgal ramps are the most common facies type in the upper Miocene of the Mediterranean and the middle Miocene of the Gulf of Suez and Red Sea, whereas the lower Miocene of the Middle East is characterized by low-energy, shallow shelfal and lagoonal carbonates. Under arid climatic conditions, recharge of fresh groundwaters is limited, thus minimizing meteoric dissolution and cementation during subaerial exposure. Instead, sea level fall and lowstand commonly resulted in extensive evaporitic conditions. Hypersaline brines, originated within evaporitic basins or lagoons, were the main cause of the pervasive dolomitization, leaching of metastable skeletal grains, and generation of moldic, vuggy, and intercrystalline porosity. Often associated with these arid landlocked carbonates is the widespread anhydrite cement that precipitated over a wide range of burial depths. Porosity distribution in these carbonates thus depends not only on the original depositional fabrics but more importantly on the degree to which the porosity-occluding anhydrite cement was dissolved or the low-permeability carbonates were fractured. These Miocene examples illustrate the contrasting diagenetic patterns and reservoir styles controlled by paleoclimatic and paleoceanographic trends and demonstrate the importance of the often overlooked hypersaline diagenesis in arid, land-locked subtropical-temperate settings. The development of exploration and production strategies for carbonates that were deposited in different climatic settings should acknowledge and use these fundamentally different styles of diagenesis and reservoir characteristics.