Multiscale Characterization of Fracture Patterns: A Case Study of the Noble Hills Range (Death Valley, CA, USA), Application to Geothermal Reservoirs

Abstract

In the basement fractured reservoirs, geometric parameters of fractures constitute the main properties for modeling and prediction of reservoir behavior and then fluid flow. This study aims to propose geometric description and quantify the multiscale network organization and its effect on connectivity using a wide-ranging scale analysis and orders scale classification. This work takes place in the Noble Hills (NH) range, located in the Death Valley (DV, USA). The statistical analyses were performed from regional maps to thin sections. The combination of the length datasets has led to compute a power law exponent around −2, meaning that the connectivity is ruled by the small and the large fractures. Three domains have been highlighted in the NH: (1) domain A is characterized by a dominance of the NW/SE direction at the fourth order scale; (2) domain B is characterized by a dominance of the E/W and the NW/SE directions at respectively the fourth and third order scales; (3) domain C is also marked by the E/W direction dominance followed by the NW/SE direction respectively at the fourth and third order scale. The numerical simulations should consider that the orientation depends on scale observation, while the length is independent of scale observation.

Fluid-Rock Interactions in a Paleo-Geothermal Reservoir (Noble Hills Granite, California, USA). Part 2: The Influence of Fracturing on Granite Alteration Processes and Fluid Circulation at Low to Moderate Regional Strain

“…Thus, Section 4.1 of this study will give new elements, improving the observations made by [38,41] concerning the NH structure. A precise fracture pattern characterization of the NH was performed by [42], through a wide-ranging analysis scale from the microscopic scale to the regional scale. These authors showed that the NH fracture network geometry has been controlled by the SDVFZ and the GFZ.…”

Fluid-Rock Interactions in a Paleo-Geothermal Reservoir (Noble Hills Granite, California, USA). Part 1: Granite Pervasive Alteration Processes away from Fracture Zones

“…It increases in intensity where the NH range converges with the Avawatz mountains ( [38] and references therein), at the intersection between the SDVFZ and the GFZ [30]. Chabani et al (2021) [12] have identified E-W structures in this area and they suppose that these structures are linked to the activity of the GFZ and the convergence of the Avawatz and the NH range. It is tempting to propose that this convergence implied the elevation of the topography at the southern part of the NH range.…”

“…In that case, the magmatic texture is preserved. These faults are striking mainly NW/SE (SDVFZ direction) ( [12], this issue). In order to be more precise in the sample selection, this latter was also based on the degrees of microfracturing defined by [44], from microscopical observations.…”

“…It consists in the number of fracture intersects per line length of scanline. The average was calculated to obtain a value of fracture density attributed to each thin section mosaic and corresponding to each microfracturing degree defined by [44]: F d 0 < 1687 fracs/m-no to very low microfracturing F d 1 = 1687 fracs/m-microfracturing of order less than the grain size F d 2 = 2694 fracs/m, with a multiplicator factor of 1.6-microfracturing of grain size order with interconnections F d 3 = 3549 fracs/m, with a multiplicator factor of 1.3-abundant microfracturing F d 4 ≥ 5140 fracs/m ( [12] this issue), with a multiplicator factor of 1.4-very abundant microfracturing Samples selected for this study have so a fracture density lower or equal to F d 2.…”

“…The study is based on the analysis of (1) rock mineralogical and related petrophysical properties changes and (2) associated chemical transfers between host rock and percolating fluids. All samples have been collected away from fractures described by [12] (this issue) in order to avoid the influence of strain and massive fluid flow. This sampling strategy ascertains the preservation of the protolith initial magmatic texture and mineralogy.…”