How long does it take to make a natural fracture?

(Press-News.org) Boulder, Colo., USA – How long does it take for natural Earth processes to form hydraulic fractures? Is the formation driven by sediment compaction, oil and gas generation, or something else? What role do these natural fractures play in modern hydraulic fracturing production? A new GSA BULLETIN study by András Fall and colleagues from The University of Texas at Austin, Virginia Tech, and ExxonMobil addresses these questions, and the article is open-access online.

The process of fracture formation by a natural increase in pore-fluid pressure has previously been referred to as natural hydraulic fracturing. Researchers work to understand these fractures through examination of fluid inclusions trapped in minerals within the fractures. In this study, Fall and colleagues conclude that natural hydraulic fractures formed over time spans of 33 to 35 million years, driven by the slow generation of natural gas.

Natural fractures provide important pathways for the flow of water, natural gas, and oil in geologic formations, including unconventional tight-gas sandstone oil and gas reservoirs targeted for production by hydraulic fracturing. These fractures play an essential role during well completion and production by connecting pores in the reservoir rock storing oil and gas to the hydraulic fracture and wellbore that allow production. "Sweet spots," or zones of higher than average permeability, have been attributed to the presence of these open fractures.

Successful prediction of zones of increased fracture abundance provides an opportunity to minimize drilling and completion costs as well as the environmental footprint of production. Successful prediction of natural fracture occurrence and their hydraulic properties requires models of fracture formation that are based on realistic mechanical, hydraulic, and chemical principles that can be tested against core, well-log, and production data.

This study is open-access (free) online at http://dx.doi.org/10.1130/B31021.1.

FEATURED ARTICLE Natural hydraulic fracturing of tight-gas sandstone reservoirs, Piceance Basin, Colorado
A. Fall et al., Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78758, USA. Published online on 30 July 2014; http://dx.doi.org/10.1130/B31021.1. OPEN ACCESS online.

Other GSA BULLETIN articles (see below) cover such topics as 1. Deep-seated landslides, river incision, and climate change in Waipaoa Catchment, East Coast North Island, New Zealand;
2. Ancient water (meteoric water) in Snake Range rocks, Nevada, USA; and
3. A seismotectonic model that accounts for most of historical and instrumental seismicity in the Po Plain of northern Italy.

GSA BULLETIN articles published ahead of print are online at http://gsabulletin.gsapubs.org/content/early/recent; abstracts are open-access at http://gsabulletin.gsapubs.org/. Representatives of the media may obtain complimentary copies of articles by contacting Kea Giles.

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Hillslope response to climate-modulated river incision in the Waipaoa Catchment, East Coast North Island, New Zealand
E.L. Bilderback, Geologic Resources Division, National Park Service, P.O. Box 25287, Denver, Colorado 80225, USA. Published online on 30 July 2014; http://dx.doi.org/10.1130/B31015.1.

Hillslope response to climate-modulated river incision in the Waipaoa Catchment, East Coast North Island, New Zealand investigates how hillslopes respond to river incision and climate change in an uplifting landscape over a glacial-interglacial cycle. High-resolution topographic data sets combined with field mapping and tephrochronology indicate that hillslopes adjusted to rapid river incision through the initiation and reactivation of deep-seated landslides. Deep-seated landslides can occupy over 30% of the surface area of the areas studied. Air fall tephra from multiple volcanic eruptions allows the timing of the onset of widespread hillslope adjustment to be bracketed between circa 14,000 and 9,500 years ago. Using results from landform tephrochronology and mapping, a conceptual time series of hillslope response to uplift and climate change-induced river incision over the last glacial-interglacial cycle is presented.

Meteoric water circulation in a rolling-hinge detachment system (Northern Snake Range core complex, Nevada)
A. Gébelin et al., Biodiversity and Climate Research Centre (BiK-F) and Senckenberg, Senckenberganlage 25, 60325 Frankfurt/Main, Germany. Published online on 30 July 2014; http://dx.doi.org/10.1130/B31063.1.

Old water in deformed rocks documents how the Nevada mountains formed in the past. Analysis of the composition of surface-derived fluids shows that water sourced at high elevation was able to penetrate to deep levels of the continental crust. So-called metamorphic core complexes in the Basin and Range of Nevada contain zones of deformation that channeled these waters and hence represent robust archives for (paleo-) fluid flow. In the case of the Snake Range, such fluid flow was intimately coupled to the permeability structure in the upper crust and was associated with active faults that over time were rendered inactive during exhumation.

Evidence for late Alpine tectonics in the Lake Garda area (northern Italy) and seismogenic implications
G. Scardia et al., Instituto Oceanográfico, Universidade de São Paulo, Praça do Oceanográfico 191, 05508-120 São Paulo, SP, Brazil. Published online on 30 July 2014; http://dx.doi.org/10.1130/B30990.1.

The Po Plain in northern Italy is a densely inhabited area, characterized by low deformation rates and damaging earthquakes with millennial‐scale recurrence time. By integrating seismic reflection data and detailed geologic mapping it has been possible to document the Plio-Pleistocene tectonic evolution of the Southern Alps. Stratigraphic and structural data allowed for the definition of three main tectonic events and new active faults have been identified. All the gathered data have been integrated in a seismotectonic model, which accounts for most of historical and instrumental seismicity in the Po Plain.

Detrital zircon provenance of the Late Cretaceous-Eocene California forearc: Influence of Laramide low-angle subduction on sediment dispersal and paleogeography
G.R. Sharman et al., Dept. of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA. Published online on 30 July 2014; http://dx.doi.org/10.1130/B31065.1.

Glenn R. Sharman and colleagues present a synthesis of new and existing radiometric ages of individual sand grains (zircon) from sandstone that was deposited between 100 and 40 million years ago in ocean basins along the western margin of North America (Oregon, California, and Baja California). Their study shows how the source of the sand changed over time in response to plate tectonics. In particular, an episode of low-angle plate subduction and associated oceanic plateau collision with southern California had a major influence on how landscapes evolved along the western margin of North America. For example, Sharman et al. show that rivers that emptied into the paleo-Pacific ocean migrated eastward over time in response to a redistribution of topography along the margin following oceanic plate collision.

Timing and significance of gabbro emplacement within two distinct plutonic domains of the Peninsular Ranges batholith, southern and Baja California
D.L. Kimbrough et al., Dept. of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA. Published online on 30 July 2014; http://dx.doi.org/10.1130/B30914.1.

Evaluating the crucial role postulated for mantle-derived mafic magmas in the formation of Cordilleran continental batholiths requires well-determined igneous crystallization ages from gabbro intrusions as well as petrologic data reflecting possible mantle source heterogeneities and/or variations in subcrustal processes. This paper by David L. Kimbrough and colleagues establishes the chronological framework for gabbro emplacement in one of the best known Cordilleran batholith of the Americas, the Peninsular Ranges batholith southern and Baja California, and thereby contributes to fundamental questions related to the formation of continental crust.

Three-dimensional (3-D) finite strain at the central Andean orocline and implications for grain-scale shortening in orogens
N. Eichelberger, Dept. of Geosciences, Princeton University, Princeton, New Jersey 08544, USA; and N. McQuarrie, Dept. of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA. Published online on 30 July 2014; http://dx.doi.org/10.1130/B30968.1.

The magnitude of contraction, or shortening, accommodated in active mountain ranges is a critical observation for understanding how mountain belts evolve over geologic time. Shortening is often estimated from cross-section models that are constrained by geologic map patterns and fault geometries at scales of kilometers or greater. In addition, micro-scale strain data from several ranges show that regionally significant amounts of shortening can also occur at the grain-scale (millimeters or less). Here, Nathan Eichelberger and Nadine McQuarrie present new micro-scale strain data from the Bolivian Andes that indicates that micro-scale shortening was a negligible part of the deformation budget. By comparing variables such as deformation temperature and rock strength between mountain ranges, Eichelberger and McQuarrie explore what factors may have limited grain-scale shortening in the Bolivian Andes. It appears that low deformation temperatures and abundant-but-weak shale favored the development of faults rather than grain-scale shortening. Globally, this implies that pre-deformation basin sedimentology of may later influence mountain belt evolution.

Age and provenance of the Cryogenian to Cambrian passive margin to foreland basin sequence of the northern Paraguay Belt, Brazil
B. McGee et al., Centre for Tectonics, Resources and eXploration (TRaX), School of Earth and Environmental Sciences, B09, Mawson Building, The University of Adelaide, Adelaide, SA 5005, Australia. Published online on 30 July 2014; http://dx.doi.org/10.1130/B30842.1.

This article by Ben McGee and colleagues presents data from a mountain belt in central-western Brazil that indicate that collision between local tectonic plates and subsequent mountain building occurred later than previously thought. The Paraguay Mountain Belt in central South America developed in response to the collision between the Amazonian Craton, the São Francisco Craton and the Paranapanema Block. The alleged ~620 million year age of mountain building has recently been questioned by geologic data that indicate the closing stages of mountain building occurred well into Cambrian time. The timing of deposition and source areas for these sedimentary rocks overlying the Amazonian Craton are investigated here using integrated U-Pb and Hf isotope data of small minerals called zircons from rocks within this sequence. Ben McGee and colleagues analyzed 742 detrital zircon ages from samples taken from the base to the top of this sedimentary succession. Maximum depositional ages from the uppermost part of this sequence of rocks, the Diamantino Formation, indicate that final deposition began no earlier than 560 (plus or minus 13) million years ago and possibly as young as the Cambrian. Given that zircon inheritance in these rocks continues up until this age and that known Amazonian Craton ages are older than ~950 million years, McGee et al. consider other potential sources for these sediments. This is achieved by integrating the U-Pb detrital zircon data with Hf isotopic data from these zircons. The Hf signature is consistent with a predominantly Amazonian source until the early-Neoproterozoic at which point the signal becomes significantly more evolved. These data, when combined with other evidence discussed here, are consistent with an ocean to the east of the present-day Amazonian Craton that didn't close until the latest Ediacaran/Cambrian.

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