Lifting the dolomite veil on legendary Arab-D super-permeability

Multiple lithofacies produce at super-k rates (at least 500 barrels of fluid/day/foot) in the Arab-D but debatable is the rock fabrics analysis responsible for the anomalous production or injection rates. Prior core work (Meyer et al, 2000) established that more than half the well with sustained super-k production in Hawiyah produce at these phenomenal rates from dolostone intervals believed to form stratiform high porosity layers sandwiched between low porosity and permeability layers. Little is known about their precursor depositional texture or anything about previous diagenetic overprints prior to dolomite mineralization.

Fig. 1 — Crystalline dolostone composed of inclusion-rich dolomite crystals banded by limpid rims supports a system of intercrystalline and vug pore types. Blue elastic fills porosity. Pain polarized light, blue plastic impregnates porosity, HWYH Well D, 7696.8 ft. core depth

Petrography

Figure 1 reproduces a photomicrograph of a thin section cut from the central super-k dolostone horizon (6050-56 ft) of Hawiyah well D (Meyer et al 2000, FIG 7, P. 367). It features a collection of cloudy crystals with clear rims. The majority of the sectioned dolomite rhombs display embedded calcite inclusions. The body of embedded particles outline microporous peloid grains. Peloids are grains with polygenetic origins (Adachi et al, 2004). Microporosity is well-developed in most Arab-D grains either during deposition or early diagenesis (Cantrell and Hagerty, 1999). Ghosted peloids display small scale heterogeneity in packing arrangements. Isolated grains seem overly uncompacted whereas clustered grain form overly compacted groupings. This variation in grain distribution is not readily explained by sedimentary processes alone. Dunham (1962, Plate 2) visualized grain support observed in two dimensions for variously sizes and shaped grains. Sections of the impregnated experimental grain supported sediment shows the number of point contacts decreases with grain size and shape complexity. Spherical plastic beads produced the greatest frequency of point contacts seen in two dimensions yet there were many grans that seemed unsupported and "floating". Peloids preserved as calcite inclusion groupings are round grains with roughly spherical shapes. Seeing many peloid ghosts that appear to float free suggests a former grain-supported framework. Certainly, the fitted contacts require grain support for corrosion localized at grain contacts.

Fig. 2 Magnified view of Arab-D dolostone thin section photomicrograph focused on heterogeneous packing arrangement of entombed peloids. Individual ghosted peloids dominate the lower left quadrant whereas compacted peloids with faced and fitted contacts prevail in the upper half of field of view.  Limpid rims of varying breath completely or partially band the crystals. Limpid dolomite rims are best developed in uncompacted areas (Meyer, 1998)

 Figure 2 shows a part of the thin section where the ghosted peloids form a mosaic of faced and fitted contacts. The mosaic of contacts shown is particularly good evidence that compaction is real instead of an illusion caused by chance. Any single fitted or faced contact might result from chance juxtaposition, although this is fairly unlikely; but for a mosaic of fitted and face contact to results from chance juxtaposition is practically impossible. Not only would each of the several involved grains have to fall into chance, but the simple unlikely happenings would have to occur in successive falls at almost the same spot. It seems evident the peloid sediment was not mud but grain supported and that it underwent compaction that is localized at grain contacts. 

Ghosted peloids display small scale heterogeneity in packing arrangements. Isolated grains seem overly uncompacted whereas clustered grain form overly compacted groupings. This variation in grain distribution is not readily explained by sedimentary processes alone. Dunham (1962, Plate 2) visualized grain support observed in two dimensions for variously sizes and shaped grains. Sections of the impregnated experimental grain supported sediment shows the number of point contacts decreases with grain size and shape complexity. Spherical plastic beads produced the greatest frequency of point contacts seen in two dimensions, yet many grains appear to “floating" as point contacts existed elsewhere in 3-d space. Peloids preserved as calcite inclusion groupings are round grains with roughly spherical shapes. Seeing many peloid ghosts that appear to float free suggests a former grain-supported framework. Certainly, the fitted contacts require grain support for corrosion localized at grain contacts. Figure 2 shows a part of the thin section where the ghosted peloids form a mosaic of faced and fitted contacts. The mosaic of contacts shown is particularly good evidence that compaction is real instead of an illusion caused by chance. Any single fitted or faced contact might result from chance juxtaposition, although this is fairly unlikely; but for a mosaic of fitted and face contact to results from chance juxtaposition is practically impossible. Not only with each of the several involved grains have to fall into chance, but the simple unlikely happenings would have to occur in successive falls at almost the same spot. It seems evident the peloid sediment was not mud but grain supported and that it underwent compaction that is localized at grain contacts. 

Figure 3.— Overly compacted Smackover Ooid (o) grainstone. White arrows point to with multiple sutured grain contacts reflecting corroded produced at grain contacts by to pressure solution.

Pressure solution may be localized at grain contacts (Fig 3) seems significant that the corrosion affecting the ghosted Peloids from the super-k dolostone in Hawiyah do not display more grain contacts that carry the weight of the overburden. Pressure solution, however, cannot account for the Hawyiah heterogeneous grain contact distribution of peloid corrosion. 

Up to half of the entombed display multiple grain contacts in the upper half of figure 2. A tightly packed group of grains is recorded from compacted carbonate sand in modern beach rock. Smith (2015) attributed similar fabrics as indicative of a vadose setting. The preserved heterogeneous character of compaction is consistent with grain reorientation and unsutured chemical corrosion associated with a vadose diagenetic setting. 

Discussion

Eugenic Karst development

Limestones sands in a eogenetic karst setting still retain some aragonite and, even, some Mg calcite (Mylroie and Vacher, 1999). The hydrology of the lenses occupying the limestones, occurs in rocks that are still in the general area of their formation. Rearangement of grains such as seen in vadose compaction increase the hydrologic activity during exposure. Increased hydrologic conductivity and channeled in isolated passages.

Seawater makes up the burial fluids in most marine carbonate deposits. Its high density relative to meteoric fluids necessitates strong flow conditions for effective flushing by meteoric water (Einsele, 2000). Progressive channeling of meteoric water in the eogenetic realm flushes burial fluids (seawater). 

Dolomite precipitation 

When CO2 saturated meteoric grounder mix with seawater, the resulting solution becomes undersaturated for calcite, but is still supersaturated with respect for dolomite (Bodiozomani 1979). Under these conditions limestone (lime sand) dissolution may be accompanied by dolomite precipitation provided the presence of suitable organic matter (Roberts, et al, 2013). Fresh groundwater may partially flush out primary marine pore water.  Recharge and the proximity of older lithified limestone datermine the plume of mixed meteoric and sea water. A low rate of recharge is presumed given the inferred arid setting of Arab-D deposition. Recent reactive transport modeling establishes the hydrology of water masses associated with dolomite formation (Lu and Cantrell 2016). Mapping of non stratigraphic abiotic dolomite shows results consistent with the idea of dolomite bodies developed via fluid movement sourced parallel to inferred paloecoastlines.

Conclusions

1. Vadose compaction features document eogenetic karst development in the Arab-D.
2. Eogenic karsting had a large meteoric coastal lens developed that paralleled the exposed paleo shoreline 
3. Vadose compaction increased sediment stiffness and facilitate increased hydrologic conductivity and channeled via isolated passages.
4. Solution pipes confined and focused mixing zone fluids 
5. Dolomite precipitation froze the eogenic hydraulic pathways
6. Dolomite frozen karst channels form the conduits capable of super permeability
7. High dolostone porosity developed in the mixing zone plume sustains super k production

References

Adachia, N., Ezakia, Y., and Liu, J., 2004, The fabrics and origins of peloids immediately after the end-Permian extinction, Guizhou Province, South China: Sedimentary Geology,, v. 164, no. 1-2, p. 161-178.

Badiozamani, K., 1973, The Dorag Dolomitization Model--Application to the Middle Ordovician of Wisconsin: Journal of Sedimentary Petrology, v. 43 no. 4, p. 965-984.

Cantrell, D. L., and Hagerty, R. M., 1999, Microporosity in Arab Formation Carbonates, Saudi Arabia: GeoArabia, v. 4, no. 2. p.

Cantrell, D. L., Swart, P. K., Handford, R. C., Kendall, Christopher G. , and Westphal, i., 2000, Geology and Production Significance of Dolomite, Arab-D Reservoir, Ghawar Field, Saudi Arabia: GeoArabia,, v. 6, no. 1, p. 45-60. 

Cantrell, D., Swart, P. K., and Hagerty, R. M., 2004, Genesis and characterization of dolomite, Arab-D Reservoir, Ghawar field, Saudi Arabia: GeoArabia, v. 9, no. 2, p. 11-36.

Dunham, R. J., 1962, Classification of carbonate rocks according to depositional texture, in W. E. Ham, ed., Classification of Carbonate Rocks, in Ham, W. E., ed., Classification of Carbonate Rocks, American Association of Petroleum Geologists, Memoir 1, p. 108-121.

Einsele, G., 2000, Sedimentary basins : evolution, facies, and sediment budget, Berlin ; New York, Springer, xi, 792 p. p.

Lu, P., and Cantrell, D., 2016, Reactive transport modelling of reflux dolomitization in the Arab-D reservoir, Ghawar field, Saudi Arabia: Sedimentology, v. 63, no. 4, p. 865–892.

Meyer, F. O., Price, R. C., and Al-Reimi, S. M., 2000, Stratigraphic and petrophysical characteristics of cored Arab-D super-K in- tervals, Hawiyah area, Ghawar field, Saudi Arabia: GeoArabia, v. 5, p. 355-384.

Mylroie, J. E., and Vacher, H. L., A conceptual view of carbonate Island Karst, in Palmer, A. N., Palmer, M. V., and Sasowsky, I. D., eds., Karst Modelng: proceedings of the symposium held February 24 through 27, 1999, Charlottesville, Virginia: Charles Town, W. Va., Karst Waters Institute, p. 48-57

Roberts, J. A., Kenward , P. A., Fowle , D. A., Goldstein , R. H., Gonzalez , L. A., and Moore , D. S., 2013, Surface chemistry allows for abiotic precipitation of dolomite at low temperature Proc. Natl. Acad. Sci. U.S.A. , v. 110, no. 36, p. 14540-14545.

Smith, L. B., 2015, Fitted-fabric grainstones - evidence for vadose diagenesis, AAPG Annual Convention and Exhibition: Denver, aapg.