ABSTRACT: Spatial Overlap of Diagenetic Events in Carbonate Rocks: Impact on Reservoir Geometry and Petrophysical Properties

LUCIA, F. JERRY, Bureau of Economic Geology, Austin, TX

Carbonate reservoir geometry and petrophysical properties are products of depositional and diagenetic events. Whereas depositional events are spatially unique, diagenetic events commonly overlap in space. Each overlapping diagenetic event modifies the previous event so that information concerning the sequence and nature of diagenetic events is necessary to define the geometry and petrophysical characteristics of the end result.

In Holocene carbonate sediments, porosity and permeability are controlled by depositional texture. With time and burial, pore sizes are gradually reduced by cementation and compaction processes, with mud-supported sediments losing porosity and permeability more rapidly than grain-supported sediments. Despite this trend of facies-controlled reduction in porosity and permeability with burial, some grain- and mud-supported limestones of Jurassic age are still permeable. In upper Paleozoic reservoirs, however, only grain-supported limestones tend to be permeable, and in lower Paleozoic strata all limestones tend to be impermeable.

The distribution of porosity and permeability in limestones at the time of dolomitization is a major controlling factor in the geometry and petrophysical properties of dolostones. Evaporative tidal flats are sources of dolomitizing water that produces syndepositional dolostones. The reflux system, however, can extend downward hundreds of feet to dolomitize strata millions of years old, strata that may have undergone extensive previous diagenesis. Permeabilities greater than 1 md in lime-mud-dominated dolostones as old as Jurassic age suggest that large volumes of dolomitizing fluids can flow through limestones 10 to 100 m.y. old and produce extensive dolostones. After further loss of porosity, however, the only conduits for dolomitizing waters may be fractures, caverns, and collapse b eccias.

Studies of Cenozoic carbonates suggest that dolomitization does not create porosity but rather is a porosity-destroying mechanism. Thus, dolostones are no more porous, and are usually less porous, than the precursor limestone. With burial, dolostones retain porosity better than limestones because they are less compactable. Porosity can be significantly reduced by overdolomitization during the initial dolomitization event and by dolomite cement during overlapping dolomitization events. Dolomitized mud-dominated sediments commonly have a crystal size ten times larger than that of the limestone precursor, resulting in an increase in pore size.

Tectonism and eustasy can result in exposure of carbonates for varying lengths of time. Long periods of exposure can produce extensive caverns, which collapse to form fractures and collapse breccias. Collapse and fracturing occur long after cavern formation and should be considered separate but overlapping diagenetic events. The resulting touching-vug pore networks can provide the conduits for later diagenetic fluids, resulting in overlapping of earlier diagenetic events.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)