Constraining Diagenesis Using Fluid Inclusions in Cements from Petroleum Basins: Examples from North Sea and Middle East

T. F. Tsui, R. K. Nishimori, R. B. Koepnick, J. D. Cocker

Cements in petroleum reservoirs commonly contain inclusions filled with brine, hydrocarbon, or mixtures of both. The physicochemical conditions of cementation as well as the quality of the inclusion oil can be derived from microthermometric and fluorescence microspectrophotometric analysis of fluid inclusions. These data, when integrated with petrographic, geochemical, engineering, and burial history data, can constrain the timing and nature of both diagenetic events and oil migration.

Brine and oil-bearing inclusions occur in quartz overgrowths from the Brent Sandstone, Northwest Hutton field, North Sea. Fluorescence analysis indicates that the inclusion oil is similar to the reservoired oil. Inclusion data define the reservoir conditions that existed during the initial stages of oil accumulation and concurrent quartz cementation. The abnormally high pressure of inclusion entrapment suggests that the reservoir was overpressured. Paleo-overpressuring probably helped preserve reservoir porosity by slowing compaction-related loss of porosity in coarse-grained reservoir sands.

Brine, oil, and three-phase (brine, oil, and vapor) inclusions occur in burial-related calcite and dolomite cements from a Lower Cretaceous (Thamama) carbonate reservoir in the Middle East. Inclusion data indicate that these cements were precipitated from and simultaneously trapped an invading basinal brine that contained an immiscible component of hydrocarbon. The intersection of brine and oil isochores in pore-throat space allows a unique determination of the time and depth of initial reservoir fill-up. The fill-up appears to coincide with the onset of peak oil generation from the source rocks believed to have supplied these reservoirs.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.