Diagenesis and porosity development of paleozoic carbonate and early to mid cretaceous siliciclastic reservoir intervals, Hopedale Basin, Labrador Shelf

Schwartz, Stephen Samuel (2008) Diagenesis and porosity development of paleozoic carbonate and early to mid cretaceous siliciclastic reservoir intervals, Hopedale Basin, Labrador Shelf. Masters thesis, Memorial University of Newfoundland.

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Abstract

Paleozoic carbonates and siliciclastic basins Early to Mid Cretaceous sandstones are observed in the Hopedale and Saglek and represent prerift and synrift sedimentation prior and during the development of the Labrador Shelf, respectively. Both of these sediments have been (and are) of hydrocarbon importance, particularly acting as regional reservoirs, with exploration drilling commencing in the 1970's and ending in the early to mid 1980's. The result was 27 exploration wells and more than 100 000 line km of seismic data acquired from the Canadian side of the Labrador Sea. Hydrocarbons, existing mainly as gas and gas condensates, have been discovered in 5 wells in the Hopedale Basin: Snorri J-90, Hopedale E-33, North Bjarni F-06, Bjarni 0-82, and Gudrid H-55. Reserves are estimated at 4.2 Tcf of natural gas and 123 million barrels of NGL. Play types include both siliciclastic (e.g. North Bjarni) and carbonate reservoirs (e.g. Gudrid H-55). North Bjarni has reserves at 2.2 Tcf of gas and 82 million barrels of NGL. -- The Hopedale Basin carbonates are represented by dolomites and limestones, and were investigated in cores from three exploration wells: Indian Harbour M-52, Gudrid H-55, and Roberval K-92. Core samples from Roberval K-92 (3578 - 3582 m and 3870 - 3873.5 m), Gudrid H-55 (2676 - 2680 m), and Indian Harbour M-52 (3952 - 3958 m) were collected for petrographic investigations and geochemical analyses. Two main types of carbonates are recognized; a wackstone with limited dolomitization (Indian Harbour M-52) and dolostone (Roberval K-92 & Gudrid H-55). Three dolomite phases (Dl, D2 and D3) have been petrographically and geochemically identified and correlated between wells, indicating diagenesis as a result of progressive burial. Trace element and stable isotope geochemistry, along with fluid inclusion data, support petrographic data and aid in the understanding the origin and nature of the dolomitizing fluids. All porosity is recognized as secondary and is a result of several stages of dissolution and dolomitization. However, precipitation of calcite cements has occluded porosity, creating reservoir heterogeneity. ⁸⁷Sr/⁸⁶Sr values, sampled from micritic calcite and dolomites, suggest that deposition likely started in the Early to Mid-Ordovician with early dolomitization occurring no later than this time. -- The Bjarni Formation represents the main siliciclastic reservoir on the Labrador Shelf and is composed mainly of fluvial to alluvial sandstones, with interbedded to interlaminated lacustrine shales. Six exploration wells from different stratigraphic depths and locations within the Hopedale Basin, containing approximately 31 m of Bjarni Formation sandstone, were logged, sampled, and petrographically analyzed from intervals that include Bjarni H-81 (2157-2164 m), Ogmund E-72 (2234 - 2240 m), North Bjarni F-06 (2452 - 2458 m), Herjolf M-92 (2632 - 2640 m), Roberval K-92 (3095 - 3112.5 m), and North Leif 1-05 (3110 - 3113.5 m). Petrographic and core analysis have identified compositionally and texturally varied sandstones, ranging from Q₉₀.₅F₂.₀L₇.₇ to Q₃₈.₅F₃₉.₀L₂₂.₅, that contain diagenetic minerals such as iron oxides (e.g. hematite), chlorite, ferroan and non-ferroan calcite cements, kaolinite, and quartz overgrowths/cements. Both intergranular (initial) and intragranular (secondary) porosities are recognized and are a result of sediment maturity and diagenesis, respectively. Diagenetic minerals are observed occluding mainly intergranular porosity reducing the total porosity, while the intragranular porosity, present in all studied intervals, supplements the total porosity, improving the reservoir quality.

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