Structurally controlled hydrothermal dolomite (HTD) reservoir facies and associated productive leached limestones are major hydrocarbon producers in North America and are receiving increased exploration attention globally. They include multiple trends in the Ordovician (locally, Silurian and Devonian) of the Michigan, Appalachian, and other basins of eastern Canada and the United States, and in the Devonian and Mississippian of the Western Canada sedimentary basin. They also occur in Jurassic hosts along rifted Atlantic margins, in the Jurassic–Cretaceous of the Arabian Gulf region and elsewhere. Hydrothermal dolomitization is defined as dolomitization occurring under burial conditions, commonly at shallow depths, by fluids (typically very saline) with temperature and pressure (T and P) higher than the ambient T and P of the host formation. The latter commonly is limestone. Proof of a hydrothermal origin for HTD reservoir facies requires integration of burial-thermal history plots, fluid-inclusion temperature data, and constraints on timing of emplacement. Hydrothermal dolomite reservoir facies are part of a spectrum of hydrothermal mineral deposits that include sedimentary-exhalative lead-zinc ore bodies and HTD-hosted Mississippi Valley–type sulfide deposits. All three hydrothermal deposits show a strong structural control by extensional and/or strike-slip (wrench) faults, with fluid flow typically focused at transtensional and dilational structural sites and in the hanging wall. Transtensional sags above negative flower structures on wrench faults are favored drilling sites for HTD reservoir facies. Saddle dolomite in both replacive and void-filling modes is characteristic of HTD facies. For many reservoirs, matrix-replacive dolomite and saddle dolomite appear to have formed near-contemporaneously and from the same fluid and temperature conditions. The original host facies exerts a major influence on the lateral extent of dolomitization, resultant textures, pore type, and pore volume. Breccias, zebra fabrics, shear microfractures, and other rock characteristics record short-term shear stress and pore-fluid-pressure transients, particularly proximal to active faults. High-temperature hydrothermal pulses may alter kerogen in host limestones, a process designated “forced maturation.” Basement highs, underlying sandstone (and/or carbonate?) aquifers (probably overpressured), and overlying and internal shale seals and aquitards also may constrain or influence HTD emplacement. Although many questions and uncertainties remain, particularly in terms of Mg and brine source and mass balance, recognition and active exploration of the HTD play continues to expand. Increasing use of three-dimensional seismic imagery and seismic anomaly mapping, combined with horizontal drilling oblique to linear trends defined by structural sags, helps to reduce risk.

Structurally controlled hydrothermal dolomite reservoir facies: An overview

Integrated structural-petrographic-magnetic-basin modeling case studies in numerous foreland fold-and-thrust belts provided key information on the critical parameters and processes controlling reservoir evolution from the end of the passive margin phase to the post-orogenic collapse of the tectonic pile. Fluid-rock interactions in reservoir rocks are intensified during tectonic events, as tectonic compaction in the foreland and development and re-opening of fracture systems in the allochthon help remobilizing basinal fluids, to squeeze-out host-rock buffered fluids as well as to reinject exotic fluids in reservoir sandstone or carbonate layers. For instance, quartz cementation in Sub-Andean foothills is dominantly controlled by Layer Parallel Shortening (LPS/tectonic compaction) in the footwall of frontal thrusts. LPS can also be inferred to cause in situ recrystallisation of mesodolomite in the Canadian Cordilleran Foreland Belt. In contrast, secondary hydrothermal dolomitization of limestone strata usually accounts for lateral migration in stratigraphic conduits in the foreland and for vertical migration of mineralizing fluids in open fractures in the allochthon, respectively. Alternatively, vuggy porosity observed in allochthonous carbonate strata in the North American Cordillera can also be interpreted to result from reservoir cooling operating in a dominantly closed fluid system during tectonic uplift and coeval erosion. Basin models can provide realistic estimates of burial-temperature history that can be compared to paleo-thermometers, such as fluid inclusions or stable isotopes, and thus provide a means to determine the relative age of cementation or dissolution episodes. Basin models can also provide fluid velocities, that can be subsequently used as critical constraints on diagenetic models at reservoir scale. Natural fluid-rock interactions induced by exotic tectonic fluids are short, no longer than one million years. As such, they constitute very good models of the long term effects of CO2 and H2 S injection and storage in natural reservoirs. The integrated quantitative appraisal approach proposed here for petroleum evaluation and reservoir prediction, also provides useful information on the overall changes in fluid flow regime and fluid velocities trough time in natural open systems, that should be used as regional boundary conditions for future reservoir storage and monitoring of acid gases in natural reservoirs.

/pdf/incidence-and-importance-of-tectonics-and-natural-fluid-5ep2rajebu.pdf

Incidence and Importance of Tectonics and Natural Fluid Migration on Reservoir Evolution in Foreland Fold-And-Thrust Belts

Deposition, diagenetic and hydrothermal processes in the Aptian Pre-Salt lacustrine carbonate reservoirs of the northern Campos Basin, offshore Brazil

Multi-scale characterization of unconventional tight carbonate reservoir: Insights from October oil filed, Gulf of Suez rift basin, Egypt

Multiple dolomitization and later hydrothermal alteration on the Upper Cambrian-Lower Ordovician carbonates in the northern Tarim Basin, China

Stabilization of early-formed dolomite: a tale of divergence from two Mississippian dolomites

The 225 bcf (original gas in place [OGIP]) Parkland A pool was discovered in 1956 with the drilling of Imperial Pacific 6-29-81-15w6. This well initially production tested at 19 mmcf gas/day and has produced more than 95 bcf of sweet gas. The Parkland play type (as defined by the Geological Survey of Canada) consists principally of fracture-associated hydrothermally karsted and dolomitized reservoirs hosted in the Upper Devonian (Famennian) Wabamun Group medial ramp carbonates resting on the axis of the Peace River arch and includes major fields such as Tangent, Teepee, and Gold Creek. Ironically Parkland itself owes little of its production to hydrothermally dolomitized carbonates; instead, most of the reservoir pore volume can be attributed to microintercrystalline porosity within a pervasive replacement microquartz (chert) that occurs at the dolomite-limestone interface. The thickness of this chert zone in the 6-29 well is 35 m and occurs near the top of the Wabamun. The origin of the chert is problematic. It postdates matrix dolomitization and crosscuts the early Tournaisian (Mississippian) Exshaw Ash but is in turn truncated by later saddle dolomite and calcite veins. The replacement chert is composed of a microporous (up to 30%) meshwork of microquartz crystals averaging 5-10 µm in size and is texturally distinct from early diagenetic chert nodules also found in the Wabamun. The two cherts are also distinguishable isotopically, with the replacement microquartz showing significantly more depleted oxygen values (22 vs. 25 d18O standard mean ocean water [SMOW]), consistent with precipitation from hot fluids having a temperature range from about 140 to 200°C. Silicification has had the greatest impact on slightly dolomitized limestone at the outer margin of the hydrothermal dolostone pod. Entrapped floating euhedral crystals of dolomite within the replacement chert show little evidence of microdissolution, suggesting that the silicic (Begin page 52) acid-charged fluid, although undersaturated with respect to the microspar lime matrix, was saturated with respect to dolomite. We postulate that hydrothermal fluids of common parentage, only marginally evolved in composition--if at all, were responsible for both dolomitization and silicification. Both replacement processes occurred in relatively rapid succession at shallow burial depths and were related to an early Tournaisian period of hydrothermal activity associated with both wrench and minor extension structuring linked with the nascent development of the Fort St. John graben. The source of the silica is thought to be the immediately subjacent Granite Wash or possibly the Precambrian basement. Stratigraphic proximity to the Granite Wash is likely a prerequisite condition for chert reservoir development within the Wabamun and serves to explain, along with significantly different hydrothermal fluid temperatures (much hotter to the west), why Parkland is such a distinct field compared to Tangent and Teepee. Hydrocarbon reservoirs that are dominantly chert-hosted are relatively uncommon, but where recognized (e.g., Monterey Formation, California) they are related to the redistribution and transformation of biogenic opal-A silica derived from diatoms, spicules, and/or radiolarians in deeper water sediments. To our knowledge, Parkland is the only hydrothermal chert reservoir that has been reported anywhere in the world.

Jeffrey J. Packard

Papers

Stabilization of early-formed dolomite: a tale of divergence from two Mississippian dolomites

A Devonian hydrothermal chert reservoir: the 225 bcf Parkland field, British Columbia, Canada