Date of Award

Spring 1-1-2014

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geological Sciences

First Advisor

David A. Budd

Second Advisor

Edmund R. Gustason III

Third Advisor

David A. Sawyer

Abstract

Thin-section analysis of the upper portion of the Late Permian Khuff-C reservoir in southern Ghawar Field revealed six microfacies, each of which was deposited on a unique setting within a carbonate ramp. The ramp was mostly flat with little topographic variations but with many local sand shoals and lagoonal depressions. The six microfacies are: (1) intertidal barren mudstone, (2) restricted subtidal peloidal-bioclastic wackestone, (3) unrestricted subtidal peloidal-skeletal mud-dominated packstone, (4) backshoal peloidal-skeletal grain-dominated packstone, (5) oolitic sand-shoal peloidal-oolitic-skeletal grainstone, and (6) foreshoal intraclast rudstones. Because shallow seas prevailed in the region, facies tract shifted constantly through time with rises and falls in base level. With a rise, the sand-shoal microfacies shifted landward and formed atop muddier subtidal or intertidal microfacies. With subsequent base-level decline, the facies tract would prograde and successively muddier and more restricted microfacies formed atop the shoal-related microfacies.

Detailed analysis of the diagenetic history of the Khuff-C reservoir showed that dolomitization is the most significant diagenetic processes followed by cementation, dissolution, neomorphism and anhydritization. Three main dolostone fabrics were identified: (1) fabric-preserving mimetic (FPM), (2) fabric-preserving non-mimetic (FPNM), and (3) fabric-destructive (FD) dolomites. With the exception of a few saddle dolomite samples, all of the fabrics above are interpreted to have occurred early during brine reflux in a marine burial setting. Stable isotope compositions of dolostones dominated by each fabric showed that averages of &delta13C for all 3 dolostones are similar, whereas average δ18O gets progressively more negative from FPM-dominated (+0.4 /) to FPNM-dominated (-0.3 /) to FD-dominated (-0.8 /). This suggests that FD-dominated dolostones formed from a less evaporated (less positive) fluid than did the FPM-dominated dolostones.

Calcite cementation is the most abundant type of cement and it occurs in at least two generations: (1st) early isopacheous rim cement interpreted to have formed on the seafloor, and (2nd) pore-filling granular mosaic cement, interpreted to have formed within a marine burial realm. A comparison between isotopic values of calcite-cemented and lime-mud rich limestones showed that the two populations are statistically similar (averages of δ13C and δ18O are 5.5 / and -3.1 / for calcite-cemented samples, and 5.3 / and -3.1 / for lime-mud rich samples). This indicates that the majority of the Khuff-C's calcite cementation and mineralogical stabilization happened at the same time from Permian seawater with shallow burial. Anhydrite, in general, post-dated previous diagenetic features but it was not necessarily the latest diagenetic features as evidenced by fractures and stylolite running through anhydrite crystals. The majority of anhydritization is interpreted to also have occurred in the marine burial realm and in advances of the refluxing dolomitizing brines, with only a small percentage of anhydrite attributed to burial diagenesis.

A number of different factors determine the amount and distribution of porosity and permeability in the Khuff-C reservoir with dolostones, in general, developing better reservoir rocks than limestones. Microporosity is a heretofore unrecognized pore system whose impact on reservoir performance needs to be evaluated.

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