Development of microporosity in a micritic limestone reservoir, Lower Cretaceous, Middle East

A compilation of average porosity and permeability data for Cretaceous petroleum reservoirs of the Middle East reveals important differences between the two main tectonic provinces. The Arabian Platform is characterized by inverse correlation of average porosity with burial depth in both carbonates and sandstones, whereas the Zagros Fold Belt (almost exclusively carbonates) has distinctly lower porosity and no depth correlation. These contrasts are suggested to reflect the fact that Arabian Platform strata are mostly near their maximum burial depth, whereas Zagros strata have experienced varying uplift and erosion following maximum burial in mid-Tertiary time. The carbonate reservoirs show no correlation between average porosity and average permeability, probably because of wide differences in the dominant pore types present, and permeabilities tend to be much higher for sandstones than for carbonates. Existence of the Arabian Platform porosity–depth correlation, despite an apparently wide diversity of depositional settings and early diagenetic porosity modifications among the individual reservoirs, illustrates and confirms some fundamental generalities about how burial diagenesis controls the overall porosity evolution of reservoir rock bodies. Although porosity commonly shows enormous small-scale heterogeneity in both carbonates and sandstones, the average pre-burial porosity of larger stratigraphic intervals tends to be very high. Burial diagenesis progressively destroys this porosity by chemical compaction and associated (stylolite-sourced) cementation. Thus, all portions of the affected rock body move toward the zero limit as depth increases, although the rates of porosity occlusion vary greatly, depending on rock fabric and early diagenesis. Average reservoir porosity therefore tends to correlate inversely with maximum burial depth, regardless of initial lithological heterogeneity.

An overview of reservoir quality in producing Cretaceous strata of the Middle East

When the highlands of Arabia were still covered with an ice shield in the latest Carboniferous/Early Permian period, separation of Gondwana started. This led to the creation of the Batain basin (part of the early Indian Ocean), off the northeastern margin of Oman. The rifting reactivated an Infra-Cambrian rift shoulder along the northeastern Oman margin and detritus from this high was shed into the interior Oman basin. Whereas carbonate platform deposits became widespread along the margin of the Neo-Tethys (northern rim of Oman), drifting and oceanization of the Batain basin started only in Late Jurassic/Early Cretaceous time. Extensional tectonics was followed in the Late Cretaceous by contraction caused by the northward drift of Greater India and Afro-Arabia. This resulted in the collision of Afro-Arabia with an intra-oceanic trench and obduction of the Semail ophiolite and the Hawasina nappes south to southwestward onto the northern Oman margin ~ 80 m.y. ago. During the middle Cretaceous, the oceanic lithosphere (including the future eastern ophiolites of Oman) drifted northwards as part of the Indian plate. At the Cretaceous–Palaeogene transition (~ 65 Ma), oblique convergence between Greater India and Afro-Arabia caused fragments of the early Indian Ocean to be thrust onto the Batain basin. Subsequently, the Lower Permian to uppermost Maastrichtian sediments and volcanic rocks of the Batain basin, along with fragments of Indian Ocean floor (eastern ophiolites), were obducted northwestward onto the northeastern margin of Oman. Palaeogene neo-autochtonous sedimentary rocks subsequently covered the nappe pile. Tertiary extensional tectonics related to Red Sea rifting in the Late Eocene was followed by Miocene shortening, associated with the collision of Arabia and Eurasia and the formation of the Oman Mountains.

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Late Palaeozoic to Neogene geodynamic evolution of the northeastern Oman margin

Bioecostratigraphy of the Shu’aiba Formation, Shaybah field, Saudi Arabia

The vertical distribution of bedding-parallel stylolites in cores from the approximately 50-m-thick Thamama-B reservoir zone of the Lower Cretaceous Kharaib Formation is compared between two wells on the water-filled flanks and two wells on the crest of a giant oilfield where oil has previously been interpreted as having preserved higher porosity on the crest. Stylolite abundances indicate division of the reservoir zone into three intervals: The top 12 ft (3.7 m) of the zone contains abundant low-amplitude stylolites in both crest and flank locations. The main, central part of the zone has abundant high-amplitude stylolites in the flank wells but nearly no stylolites in the crest. The basal 17 ft (5.2 m) of the zone contains high-amplitude stylolites in all wells, but these are somewhat more abundant in the cores from the crest of the field. The contrast in stylolite abundance in the center interval is consistent with oil having inhibited chemical compaction on the crest of the field. Minor dissolution along wispy seams in the center interval of the crest is interpreted as representing an incipient stage of stylolite development. The similarity in stylolite abundance in the top interval between crest and flank wells is interpreted as reflecting stylolite formation pre-dating oil emplacement because of greater depositional concentration of clay in thin laminations resembling flaser bedding. Stylolite growth in the basal interval may also partly predate oil filling, but the evidence for timing is unclear because stylolites in the heavily bitumen-stained basal interval of the crestal wells have thick bitumen coatings that may be residues from dissolution of the surrounding limestone. Negative correlation between porosity and both amplitude and proximity of stylolites supports the model of porosity loss by calcite cementation derived from stylolites. The overall porosity difference between crest (22%) and flanks (12–16%) approximately matches the volume of cement that would have been supplied by the thickness of strata dissolved on the flanks. The match between porosity difference and thickness variation supports a diagenetic system closed to significant calcite import or export. Profiles of bulk-chemical analyses reveal how clay (proportional to aluminum) and dolomite (proportional to magnesium) vary both vertically in the formation and laterally between the crest and flank locations. Higher clay values, suggested to facilitate stylolite development, occur mainly in the top and basal parts of the zone, reflecting episodic deposition of siliciclastic fines. Higher dolomite contents throughout the wackestone-dominated lower two thirds of the reservoir zone (around 5–10 wt. %) than in the grain-supported upper one third (little or no dolomite) are suggested to reflect differences in early dolomitization by seawater attending slower sedimentation rates in the lower part of the zone. Like the variations in chemical compaction, dolomitization is thus a manifestation of the layer-cake geometry of this reservoir. Detailed core descriptions from each well and tables of bulk-rock chemical analyses and stylolite data are available as supplemental material.

Stylolites and Porosity In A Lower Cretaceous Limestone Reservoir, Onshore Abu Dhabi, U.A.E.

The Yibal oil field in west central Oman is a large dome created by deep-seated salt movement. The maximum oil column is 370 ft (112.8 m) and the productive area is about 4.3 x 6.2 mile (7 x 10 km). The structure is complicated by extensive tensional faulting. The main oil accumulation is in the Shuaiba chalk overlain disconformably by the Nahr Umr shale. The Shuaiba reservoir is in pressure communication with the underlying Kharaib formation. It is speculated that early accumulation of oil played a major role in preserving the high porosity and greater crestal reservoir thickness by inhibiting diagenetic processes. This field study provided the reservoir simulation engineer with a viable geological and petrophysical model of the Shuaiba reservoir to aid in understanding observed phenomena.

Khalifa M. Al-Hinai

Papers

Shuaiba reservoir geological study, Yibal field, Oman