Browsing by Subject "Reservoir analog"

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Evolution of a complex tidally-modified to wave-modified deltaic reservoir analog : the Campanian Blair Formation, Mesaverde Group, Rock Springs Uplift, Wyoming
(2019-05) Belcher, Keri Leigh; Steel, R. J.; Flaig, Peter Paul
The (Campanian) Blair Formation (Fm), along the Rock Springs Uplift (a N-S, doubly plunging anticline) near the city of Rock Springs, Wyoming, comprises a stratigraphic 3rd order sequence set that included lowstand, transgressive, and highstand deposits. The Blair Fm was emplaced in shallow marine depositional settings (less than 100m of water) and contains deposits of offshore, prodelta , fluvial-dominated delta front, tidally-influenced delta front , wave-influenced delta front, and subaqueous terminal distributary channels. Delta deposits coarsen and thicken upward with variable degree of reworking by tidal(t) currents or waves(w) indicated by: sigmoids(t), single and double mud drapes(t), mud chips(t), and hummocky cross-stratification(w). No evidence of deep-water basin floor or slope deposits is present in the Blair. Additionally, the Blair is similar to other shallow marine deposits of the Cretaceous Western Interior Seaway (CWIS) that display a preferential tidal dominance during lowstand and preferential wave dominance during highstand. The Blair Fm evolves from lowstand, tidally-influenced deltaic deposits to transgressive offshore and prodeltaic deposits, and ultimately into highstand wave-influenced deltaic deposits. Local embryonic Laramide uplift and differential subsidence led to changes in the basin floor slope and geometry, likely steepening the formerly gentle, ramp-style geometry of the retroarc foreland basin, and inducing high sediment supply near local uplifts. Presence of delta-front turbidites to the north and prodeltaic mudstones in the south are indicative of the locally high sediment supply to the north. The changes in basin floor topography and/or steepening of the basinal slope are evidenced by the successive, meter-scale slumped intervals and a thickness anomaly within the middle Blair, along with amplification of tidal signatures in the lower Blair. This incipient uplift during the emplacement of the Blair may have occurred earlier (82 Ma) than the postulated earliest timing of uplift at around 80 Ma suggested by other workers. The Blair Fm is considered an unconventional (tight), heterolithic reservoir that produces mainly gas in hydrocarbon fields near the Rock Springs anticline. The lower Blair, heterolithic, tidally-influenced lowstand wedge deposits may serve as an analog for other lowstand tidal reservoirs of the CWIS, including the Frewens sandstone of the Frontier Fm
Structural diagenesis of bed-parallel and bed-normal fractures, Cretaceous Crato Formation carbonate rocks, NE Brazil
(2017-12) McKinnon, Elizabeth Anne; Laubach, Stephen E. (Stephen Ernest), 1955-
Here I show that structures exposed in the limestone quarries of the Araripe Basin provide information on the fracture porosity evolution in laminated members of early Cretaceous Brazilian interior basins and provide insight in to the Crato Formation as an analog for offshore reservoirs and its role as a semi-permeable aquitard in the Araripe Basin. Permeable pathways in the microbial laminites of the Crato Formation were enhanced first by fracturing caused by elevated fluid pressures, next by fracturing caused by increasing overburden related to burial, next by jointing caused by decreasing overburden related to uplift and finally by dissolution along joint walls. Porosity was reduced by cementation of fractures, both during and after opening, and later enhanced by dissolution of cements and included host rock within veins. Bed-parallel fractures generated by fluid overpressure may be present in well-layered regions of pre-salt source rocks assuming petroleum generation is significantly high or fluid overpressure occurred sufficiently early in the burial history of pre-salt source rocks. Bed-parallel fractures may serve as storage sites or assist in the lateral transport of fluids to vertical escape pathways. The orientation of bed-normal structures in the Crato Formation is likely different from the orientation of bed-normal structures in pre-salt source rocks due to differences in the basement framework between the Araripe Basin and offshore basins such as the Campos Basin. Open joints are unlikely in subsurface reservoirs due to their relationship with exhumation. According to speleothem growth, open and partially occluded joints in the Crato Formation likely provide permeable pathways for water between the superior and middle aquifers of the Araripe Basin. This fluid movement may be enhanced along bedding planes and/or semi-permeable, bed-parallel gypsum veins with high porosity.