Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Burial Diagenesis and Petrophysical Interpretation and Its Significance in Jurassic Carbonate Reservoir Characteristics of the Middle East
(1) Middle East Geological Establishment, Al-Ain, United Arab Emirates.
The Jurassic sequence of the Middle East which consists of deep-water mudstone and argillaceous shales (source rocks); shallow-water/high energy grainstone and packstone (reservoir facies); and very shallow supertidal anhydrite (seal facies) are forms the most petroliferous sequence in the world. Porosity in these reservoirs are mainly of primary/intergranular type in shoal carbonates, ranging from about 15 to 30%, but its structure and distribution pattern are quite complicated. Pore spaces often exist in banks of dolomitized corals and also in casts and molds of leached forams and grains. Permeability is sometimes increased by fractures. Porosity preservation may have been possible if the near-surface cementation, which could inhibit late stage pressure-solution, had taken. Presence of oil in the reservoirs may also have slowed diagenesis and thus helped to preserve primary porosity. The presence of impermeable and ductile anhydrite layers in the sequence must not be overlooked. These layers may have showed diagenesis by preventing extensive/late fresh water influx, and also retarded compaction by acting as buffer zone that absorbed part of overburden pressure. For creating secondary porosity and permeability at late burial stages, solution of carbonates, dolomitization and fracturing are the most important.
Closeness of the known areas of the accumulations and mature source rock suggests a genetic relationship between them, although complications are caused by vertical oil migration. The presence of excellent anhydrite seals is another critical factor accounting for the unmatched hydrocarbon accumulations, which have removed unbreached to the present time. Stratigraphic accumulation have been found where thin grainstone pinchout against regional dip. Major structural flanks and the basin margin area, where regional dip and facies pinchout can combine to form stratigraphic traps, provide future exploration potentials in the Jurassic. Increased subsurface contoal and improved seismic techniques have made mapping of pinchout trends in the Jurassic reservoir intervals more feasible.