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Abstract


Pub. Id: A067 (1993)

First Page: 155

Last Page: 197

Book Title: SG 36: Diagenesis and Basin Development

Article/Chapter: Eustatic and Tectonic Controls on Porosity Evolution Beneath Sequence-Bounding Unconformities and Parasequence Disconformities on Carbonate Platforms: Chapter 11: DIAGENESIS, SEQUENCE STRATIGRAPHY, AND CHANGES IN RELATIVE SEA LEVEL

Subject Group: Reservoirs--Sandstones and Carbonates

Spec. Pub. Type: Studies in Geology

Pub. Year: 1993

Author(s): J. F. Read, Andrew D. Horbury

Abstract:

Carbonate sequences and parasequences that formed under known or inferred 1 to 40 m.y. tectono-eustatic and 20 to 400 k.y. Milankovitch low to high amplitude sea level changes are shown to leave distinctive diagenetic records.

On carbonate platforms, low amplitude, high frequency sea level fluctuations typical of global green-house times form thick accumulations of meter-scale cycles with regional tidal flat caps, and there is only limited erosion of cycle tops. Humid climate cycles have early cemented, typically undolomitized intertidal fenestral caps and some supratidal laminites; aragonite fossils commonly are leached. If the climate is sufficiently arid, the cycles are dolomitized and aragonite is leached during falling sea level. Primary intergranular and moldic porosity is preserved in lower parts of cycles, and intercrystal and vuggy porosity is present in dolomitized subtidal facies and laminite caps beneath cycle top anhydrites and fine dolomite.

Moderate amplitude, high frequency fluctuations in sea level occur during times of intermediate continental glaciation. Cycles commonly have muddy facies grading up into grain-rich facies; they lack tidal flat facies and instead have small scale karstic surfaces or caliche directly over shallow subtidal facies. In humid climates, cementation plugs porosity in the upper phreatic zone, typically in grainstones just below the karstic surface. Zoned cement overgrowths in the deeper phreatic zone, although volumetrically minor, show a partial to complete record of high frequency sea level fluctuations that affected the sequence.

End_Page 155------------------------

Primary intergranular porosity is best preserved in mid-cycle packstone-grainstones and subaerial pisolitic carbonates, with moldic porosity in leached skeletal muddy carbonates. In arid climates, there is little mineralogic stabilization or sparry calcite cementation, and dolomites can form beneath offlapping evaporatic tidal flats (particularly in highstand systems tracts). Primary porosity is generally preserved with secondary intercrystalline porosity developed in dolomites.

Large amplitude, glacio-eustatic fluctuations form erosionally bounded cycles often lacking regional tidal flat facies, because sea levelfalls off the platform faster than the tidal flats can prograde. Cycle tops commonly are highly disconformable, and can have caliche, karstic sinkholes, and caves. Large scale vertical migration of marine, mixed, meteoric phreatic and meteoric vadose zones dominate platform diagenesis. In humid climates, moldic and cavernous porosity may be extensively developed extending downward through many carbonate cycles, especially in coarse, aragonitic sediments, and sparry cements and internal sediment are associated with paleowater tables. However, aquicludes in the section may cause perching of water tables, limiting meteoric diagenesis to upper regressive parts of individual cycles where pore-rimming cements preserve primary intergranular and moldic porosity against compaction. Arid climates result in extensive caliche caps and little early spar cementation (thus much primary intergranular porosity), and dolomites can form from refluxing brines.

Low frequency, 2nd- and 3rd-order sea level falls (from one to tens of m.y. duration), associated with supersequence/sequence boundaries or tectonic uplift, are the common cause of updip porosity development and regional downdip calcite cementation under humid climates. During emergence, the sediments become mineralogically stable, chalky microporosity can form, and sediments lose intergranular porosity during cementation. Aquifers evolve from diffuse flow to conduit flow with time, as karstic passages localize flow. Earlier high frequency diagenesis is overprinted. Porosity evolves from primary intergranular and moldic to vuggy and cavernous types located in cave-fill breccias and in compaction fractured cave roofs. In arid climates, reflux dolomitization may occur if the surface is overed with hypersaline brines, but otherwise diagenesis and porosity development is proportionate to rainfall and may be very slow.

These cycle styles exert a strong influence on the development of the world's largest oil and gas fields that are reservoired in platform top carbonates. In green-house times, giant fields are mainly restricted to the arid cycles; during the transition into higher amplitude ice-house times, giant fields occur in both the arid and humid cycles. High amplitude, low frequency 2nd-3rd-order emergence results in giant fields mainly if the climate is humid. Reservoir and seal success are therefore critically dependent on the relationship of climate to cycle amplitude.

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