Hydrothermal dolomites occur in Precambrian to Cenozoic strata with many models for hydrothermal dolomite emphasizing proximity to faults. Although some hydrothermal dolomites occur adjacent to significant faults, many do not. In this presentation, hydrothermal dolomite are described in three intervals and locations – Wabamun Group (upper Devonian) in western Canada, Swan Hills Formation (middle Devonian) in western Canada, and the upper Pennsylvanian at Reinecke Field in west Texas. In all three areas, petrographic and stable isotope data indicate dolomitization at high temperatures after moderate to deep burial.

Porous dolomites are surrounded by impermeable Wabamun limestones creating stratigraphic traps that are scattered across the southern Peace River Arch in western Alberta. Many hydrothermal dolomites in the Wabamun follow depositional facies and early dolomitization. Some oil fields are adjacent to mappable faults, but many are not. Many of the Wabamun fields were discovered by 3D seismic data targeting anomalies away from faults.

Hydrothermal dolomites in and around Rosevear Field in western Alberta occur in grainstones and grain-rich stromatoporoid boundstones. Adjacent micrite-rich facies are generally not dolomitized creating the stratigraphic trap at Rosevear Field. Hydrothermal brines apparently moved up into platform margin grainstones and then moved long distances along the permeable platform margin and connected embayments.

At Reinecke Field in west Texas, hydrothermal dolomites occur in an upper Pennsylvanian limestone buildup. The hydrothermal dolomites created high-permeability horizontal and vertical “raceways” within the largely limestone reservoir. Those “raceways” fundamentally affected oil production during primary, secondary and CO2 recovery at Reinecke Field.

Hydrothermal dolomites are important hydrocarbon reservoirs in many parts of the world. They have excellent reservoir characteristic because of their large crystal sizes, vugs, and fractures. Many factors other than faults can control their distribution including depositional facies, early dolomite, highly saline brines in the basin, and convective flow. Careful petrography, collecting stable isotope data, and a good understanding of the basin history can help predict these types of reservoirs in the subsurface.

The diagenetic evolution of porosity and permeability in carbonates is complex and involves a number of independent factors. Carbonate sediments start with 40-80% porosity and generally lose porosity with time and burial (Schmoker and Halley, 1982), however there are many factors that cause higher and lower porosity in carbonates of the same age and burial depth. Alteration of carbonate sediments during shallow burial is common and includes diagenesis in seawater shortly after deposition, freshwater diagenesis during subaerial exposure, and dolomitization in hypersaline waters. Marine (seawater) diagenesis varies with depth and carbonate saturation as is shown on Enewetak Atoll. Aragonite and Mg-calcite cementation dominate in shallow seawater; however aragonite is dissolved and radiaxial calcite precipitates in moderately deep seawater. In even deeper seawater, calcite dissolves and dolomite precipitates. Freshwater (meteoric) diagenesis and dolomitization commonly rearrange and decrease porosity, but they also impart strength to the rock that reduces porosity loss during deeper burial. Pennsylvanian limestones in west Texas show that prolonged subaerial exposure progressively decreases matrix porosity but increases conduit porosity (fractures and vugs), and hence, formation permeability. Reflux dolomitization is commonly associated with carbonates in arid climates like the Permian of the Permian Basin. The porosity and permeability of reflux dolomites varies according to position in the dolomitizing system with less porosity and permeability in proximal parts of the dolomitizing system. Dolomitization decreases rate of porosity loss with burial (Schmoker and Halley, 1982) allowing some porous dolomite reservoirs like the Smackover of south Alabama at depths of 16,000-18,000 feet. Deep burial dissolution increasing porosity is the exception, rather than the rule. In summary, unlike quartzose sandstones, a complex array of diagenetic factors generally affect the ultimate porosity, permeability and production of carbonate reservoirs.

West Texas and southeast New Mexico contain many classic carbonate exposures with large vertical and lateral extents that allow delineation of major sequence stratigraphic relationships. Sequence stratigraphic relationships help to predict geometries, facies, and early diagenesis in analogous systems in the subsurface. Isolated carbonate buildups are present in Mississippian and Pennsylvanian outcrops in the Sacramento Mountains, and they grew during transgressions when accommodation (relative sea level rise) was greater than or approximately equal to carbonate sediment production. Drowned isolated buildups are commonly excellent carbonate reservoirs throughout the world, including the nearby Horseshoe Atoll.

Ramp carbonates of the Permian San Andres Formation are exposed along the western side of the Guadalupe Mountains. The San Andres has a thick lower transgressive systems tract (TST) overlain by a prograding highstand systems tract (HST). Major hydrocarbon reservoirs occur in similar sequences in the subsurface. Reservoirs are commonly shelf-crest grainstones and adjacent packstones in the upper San Andres HST with structures created by differential compaction over packstone-grainstone buildups in the TST of the lower San Andres.

The Capitan Formation is part of a classic carbonate shelf system dominated by HST progradation. The same system occurs in the subsurface. The structural configuration of the prograding margin is dominated by basinward dip caused by differential compaction associated with the progradation. As a result, the fractured Capitan reef is generally structurally low and wet. Hydrocarbons occur in backreef carbonates and shelfal sands with updip, landward seals formed by impermeable lagoonal evaporites.

3D seismic data show the depositional history of shallow Pleistocene shelf margin, slope and basinal strata in offshore East Kalimantan, Indonesia. Siliciclastic sequences on the shelf are dominated by progradational packages deposited during highstands and falling eustatic sea level. During the last two lowstands of sea level (˜18 and ˜130 ka), coarse siliciclastics were generally not deposited in deep-water environments because lowstand deltas did not prograde over the underlying shelf margin. During the lowstand of sea level that ended at ˜240 ka, deltas prograded over the previous shelf edge, and sand-rich sediments spilled onto the slope.

During the late Pleistocene, siliciclastic sediment supply determined the depositional characteristics of the slope. Channel-levee complexes developed on the slope where deltaic sediment supply was large; in contrast, incised valleys/canyons formed on the slope where siliciclastic input was limited. Pleistocene channel-levee complexes can be traced upslope to lowstand deltas associated with the paleo-Mahakam River. In areas with limited sediment supply, rivers and deltas were generally not present on the outer shelf, including areas upslope from incised slope valleys and canyons. Strata on the basin floor downslope of the slope valleys and canyons are dominated by mass-transport complexes, suggesting that slope valleys and canyons formed by mass failures of the slope, not by erosion associated with turbidite sands derived from rivers or deltas.

In the area with limited sediment supply, one small river was present on the shelf margin during the upper Pleistocene, and sediments originating from its lowstand delta filled a pre-existing slope valley/canyon and formed a basin-floor fan. That slope valley/canyon has a lower fill that consists of amalgamated, sinuous channel deposits and an upper fill consisting of a shale-rich, channel-levee complex. The basin-floor fan also has two parts: a lower fan containing broad lobes with relatively continuous reflectors and an upper fan with a shale-rich, sinuous channel-levee complex that prograded over the lower fan and fed sheet-like lobes on the upper, outer fan. These shallow Pleistocene systems serve as analogs for deeper, more poorly imaged reservoir systems.