Function Of Dip Angle Research Articles

Steady-State Flow Through a Subsurface Reservoir with a Displaced Fault and its Poro-elastic Effects on Fault Stresses

We consider steady-state single-phase confined flow through a subsurface porous layer containing a displaced, fully conductive fault causing a sudden jump in the flow path, and we employ (semi-)analytical techniques to compute the corresponding pressures and fault stresses. In particular, we obtain a new solution for the pressure field with the aid of conformal mapping and a Schwarz–Christoffel transformation. Moreover, we use an existing technique to compute the poro-elastic stress field with the aid of inclusion theory. The additional resistance to fluid flow provided by a displaced fault, relative to the resistance in a layer without a fault, is a function of dip angle, fault throw divided by reservoir height, and reservoir width divided by reservoir height. Fluid flow has a larger effect on fault stresses in case of injection than in case of depletion, where injection with up-dip flow results in increased zones of fault slip near the bottom of the reservoir. Opposedly, injection with down-dip flow results in increased slip near the top of the reservoir. An order-of-magnitude estimate of the effect of steady-state flow across displaced faults in the Groningen natural gas reservoir shows that the effect on fault stresses is probably negligible. A similar estimate of the effect in low-enthalpy geothermal doublets indicates that steady-state flow may possibly play a small role, in particular close to the injector, but site-specific assessments will be necessary to quantify the effect.

Sedimentological and structural processes operative along a metalliferous catena from sandstone-hosted to unconformity-related Pb-Cu-Zn deposits in an epicontinental basin, SE Germany

The sandstone-hosted Pb-Zn deposits worldwide, including those in the Middle Triassic series of the epicontinental basin in SE Germany, were targeted upon in a literature review by Bjørlykke and Sangster in 1981, treating each mineralization as a separate entity. A different approach based on structural geology, sediment mineralogy and geochemistry (major and trace elements, C-, O-, S- and Pb isotopes) has been taken to study a “metalliferous catena” from sandstone-hosted to unconformity-related Pb-Cu-Zn deposits. The resultant model encompasses five reference types of base metal mineralization lined up along a NW-SE transect perpendicular to the various facies zones of the basin and covering the entire sequence of mineralizing processes from marine-diagenetic processes in the basin center through the epigenetic mineralization finding an outlet in the unconformity-related vein-type deposits close to the basement boundary fault at the edge of the epicontinental basin.The stratiform “Bleiglanzbank-type” (stage I) is confined to the basin center of the Middle Triassic Keuper Basin. Galena derived from bacterial reduction of sulfate and from Pb originating from a marine source of strongly homogenized average basement Pb. The Pb ore evolved in an extensional geodynamic regime. The marine-diagenetic “Bleiglanzbank-type” mineralization can be characterized as a marker stage for a Pb-enriched basin fill.The sulfidic and non-sulfidic stratabound Zn-Cu-Pb mineralization named “Weiden-type” is still dominated by Pb sulfide with only subordinate amounts of cerussite. The ore developed more landward in the epicontinental basin, in a sabkha to alluvial-fluvial depositional environment. The Pb model ages are significantly higher than those of the “Bleiglanzbank-type “ore. The reason for the chemical and facial differences in the epicontinental basin are accounted for by a barrier resultant from synsedimentary faulting around 230Ma. It is a true dia- (to epigenetic) mineralization whose fluid migration is explained by a flood recharge model. Basin dynamic and structural geology changed from extensional to moderately compressional.Two epigenetic, Pb-dominated sandstone-hosted ore types may be defined in the region, the flexurebound sulfidic and non-sulfidic Pb-(Cu-Zn) mineralization of “Eschenfelden-type” (stage III) is structure-related and superimposed on the afore-mentioned “Weiden-type” and the “Freihung-type” (stage IV). In the latter the flexure no longer existed and has turned into a reverse fault while the Pb mineralization became ore grade. The compressional regime dominated the block faulting in this ore type being located more landward. The maximum horizontal stress exerted from the basement onto the platform sediments was reached during the Lower Cretaceous resultant in a stacked pattern of Mesozoic beds and the development of an unconformity truncating the Jurassic beds. Intraformational recycling of Pb was at a maximum around the reverse faults later than 85Ma when reverse faults conduced to an overstepping of Triassic sediments, containing “Weiden-type” Pb protore (Roll front-like intraformational recycling of Pb). The 206Pb/207Pb model ages of “Weiden-“, “Eschenfeld- “and “Freihung-type” ore mineralization are almost identical. Investigations focused on the temperature of formation yielded 110° to 150°C for the sulfides and 141° to 169°C for the carbonate minerals. In this compressional regime, heat has been generated by friction along the fault planes as a function of dip angle and depth, reducing the grain size and delivering elements to the intrastratal solutions. The mineralized structures tend to strike NW-SE and in plan view are shaped concave towards the basement from which the maximum horizontal pressure was exerted onto the platform sediments.The unconformity-related sulfidic Pb-( Zn) mineralization of “Erbendorf-type” (stage V) formed in an extensional regime creating a strike-slip fault system at temperature between 132°C and 200°C. The Pb isotope signature is different from the ore types dealt with previously as far as the U series is concerned and closely resembles the sulfidic members from the sandstone-hosted Pb deposits which both have a common source for Th in monazite. Intraformational recycling in sandstone-hosted Pb deposits and concentration of Pb in unconformity –related vein-type deposits tapped the same element source and belong to the “metalliferous catena” of Pb ore crossing the basin-basement boundary or unconformity, respectively. Sandstone-hosted and unconformity-related mineral deposits are not separate entities but two sides of the same coin, the geodynamic evolution of an epicontinental basin. They are a function of the position within the “metalliferous catena” from the basin center to its edge.

Computer Simulation of Clinch Mountain Drainage Networks

Clinch Mountain, in southwestern Virginia and northeastern Tennessee, is a long homoclinal ridge with a dip angle that varies along the strike. The base width and height both decrease with increasing dip, so that the mountain provides an excellent example of a system with uniform lithology and structure but varying structural parameters. Various properties of the stream networks on both the dip and scarp slopes were measured and analyzed as a function of dip angle. A random-walk headward-growth model of stream network development was used to simulate the Clinch Mountain network; the criterion for a successful simulation was that the simulatedand actual networks should have statistical geometrical similarity. Satisfactory results were obtained if both the reduced width (product of width and drainage density) and the number of outlets per unit length were assigned as input parameters.