Adriano Mazzini

4D imaging of fracturing in organic‐rich shales during heating

To better understand the mechanisms of fracture pattern development and fluid escape in low permeability rocks, we performed time-resolved in situ X-ray tomography imaging to investigate the processes that occur during the slow heating (from 60 to 400 C) of organic-rich Green River shale. At about 350 C cracks nucleated in the sample, and as the temperature continued to increase, these cracks propagated parallel to shale bedding and coalesced, thus cutting across the sample. Thermogravimetry and gas chromatography revealed that the fracturing occurring at {approx}350 C was associated with significant mass loss and release of light hydrocarbons generated by the decomposition of immature organic matter. Kerogen decomposition is thought to cause an internal pressure build up sufficient to form cracks in the shale, thus providing pathways for the outgoing hydrocarbons. We show that a 2D numerical model based on this idea qualitatively reproduces the experimentally observed dynamics of crack nucleation, growth and coalescence, as well as the irregular outlines of the cracks. Our results provide a new description of fracture pattern formation in low permeability shales.

First sampling of gas hydrate from the Vøring Plateau

Methane hydrate is a clathrate, an ice-like solid formed from methane and water, that is stable under conditions of pressure and temperature found in most of the worlds oceans at depths greater than a few hundred meters. Hydrate occurs beneath the seabed where there is sufficient methane to exceed its solubility in water within the hydrate stability field. It has been speculated that methane released from hydrate by climate-induced changes in pressure and temperature escapes into the ocean and into the atmosphere, where its acts as a greenhouse gas. Further, methane from beneath the seabed is the primary energy source for communities of chemosynthetic biota at the seabed.

Origin and timing of sand injection, petroleum migration, and diagenesis in Tertiary reservoirs, south Viking Graben, North Sea

Petrographic, fluid-inclusion, and carbon and oxygen stable isotope studies of Tertiary injectite reservoirs in the south Viking Graben of the North Sea allow an understanding of the origin and timing of sand injection, petroleum migration, and diagenesis. Injection from shallowly (400 m; 1300 ft) buried Paleocene and Eocene unconsolidated sandstones occurred at the end of the Eocene, probably in response to earthquake activity. Liquid oil was already present in the parent sands prior to injection and leaked from the injectites to the seabed. Upward-migrating oil and basinal brines mixed with downward-invading mixed meteoric-marine pore fluids in the injectites, causing extensive biodegradation of the oil. Biodegradation of oil provided the driver for early carbonate cementation in injectites, causing diminished reservoir quality. However, early carbonate cementation also sealed off the injectites as potential escape routes for petroleum from the underlying parent sands. Oil (and gas) continued to migrate into the reservoir (parent) sands upon increased burial, causing a mixing of high-API oil with the early charged, extensively biodegraded low-API oil. The study of early diagenetic imprints reveals the evolution of injectite reservoirs, which forms the basis for understanding how to explore and develop them.

Integrated petrographic and geochemical record of hydrocarbon seepage on the Voring Plateau

Authigenic carbonate crusts, nodules and chemoherms were sampled from pockmarks and mud diapirs on the southern part of the Vøring Plateau during the TTR-8 and TTR-10 marine expeditions. A petrographic and geochemical study was carried out to investigate their possible relationship with the seepage of hydrocarbon fluids. All authigenic carbonates are depleted in 13C (−31.6‰ < δ13C < −52‰) indicating that methane is the primary source of the carbonate carbon. Furthermore, pyrite framboids are often associated with these samples, indicating that sulphate reduction is spatially coupled with methane oxidation and implying that the carbonates are formed through the anaerobic oxidation of methane. The oxygen stable isotope composition of the near-subsurface carbonates (3.1‰ < δ18O < 4.9‰) suggests a precipitation temperature very close to the one recorded on the sea floor (between −1 and 2 °C), which is consistent with their stratigraphic position, and a recent (Holocene?) age of formation. Carbonates sampled from greater depths (up to 5.5 m below the sea floor) are richer in 18O (4.6‰ < δ18O < 6.2‰), which is interpreted as a result of precipitation from an 18O-rich fluid. The occurrence of different carbonate mineral phases (aragonite, calcite, dolomite) is possibly related to varying dissolved sulphate concentrations in the diagenetic environment. Fluid inclusion microthermometry and Raman spectroscopy indicate the presence of an aqueous + hydrocarbon mixture inside the inclusions. This seepage mixture was almost certainly immiscible, resulting in heterogeneous trapping.

A 4D Synchrotron X-Ray-Tomography Study of the Formation of Hydrocarbon- Migration Pathways in Heated Organic-Rich Shale

Summary Recovery of oil from oil shales and the natural primary migration of hydrocarbons are closely related processes that have received renewed interest in recent years because of the ever tightening supply of conventional hydrocarbons and the growing production of hydrocarbons from low-permeability tight rocks. Quantitative models for conversion of kerogen into oil and gas and the timing of hydrocarbon generation have been well documented. However, lack of consensus about the kinetics of hydrocarbon formation in source rocks, expulsion timing, and how the resulting hydrocarbons escape from or are retained in the source rocks motivates further investigation. In particular, many mechanisms have been proposed for the transport of hydrocarbons from the rocks in which they are generated into adjacent rocks with higher permeabilities and smaller capillary entry pressures, and a better understanding of this complex process (primary migration) is needed. To characterize these processes, it is imperative to use the latest technological advances. In this study, it is shown how insights into hydrocarbon migration in source rocks can be obtained by using sequential high-resolution synchrotron X-ray tomography. Three-dimensional images of several immature “shale” samples were constructed at resolutions close to 5 lm. This is sufficient to resolve the source-rock structure down to the grain level, but very-fine-grained silt particles, clay particles, and colloids cannot be resolved. Samples used in this investigation came from the R-8 unit in the upper part of the Green River shale, which is organic rich, varved, lacustrine marl formed in Eocene Lake Uinta, USA. One Green River shale sample was heated in situ up to 400 � Ca s X-ray-tomography images were recorded. The other samples were scanned before and after heating at 400 � C. During the heating phase, the organic matter was decomposed, and gas was released. Gas expulsion from the low-permeability shales was coupled with formation of microcracks. The main technical difficulty was numerical extraction of microcracks that have apertures in the 5- to 30-lm range (with 5 lm being the resolution limit) from a large 3D volume of X-ray attenuation data. The main goal of the work presented here is to develop a methodology to process these 3D data and image the cracks. This methodology is based on several levels of spatial filtering and automatic recognition of connected domains. Supportive petrographic and thermogravimetric data were an important complement to this study. An investigation of the strain field using 2D image correlation analyses was also performed. As one application of the 4D (space þ time) microtomography and the developed workflow, we show that fluid generation was accompanied by crack formation. Under different conditions, in the subsurface, this might provide paths for primary migration.

Fluid escape from reservoirs: implications fromcold seeps, fractures and injected sands Part I. The fluid flow system

Abstract Fluid escape from reservoirs can take place through (hydraulic) fracturing, sand injection and seepage. Above several Tertiary hydrocarbon reservoirs in the North Sea, substantial amounts of fractures, sand injectites and seeps occur. Petrographic observations of these features show that all have carbonate cement associated with them that contains fluorescing hydrocarbon inclusions. The petroleum fluids escaping were partially trapped during the cementation and analysis of the cements allows understanding of the fluid flow system and the fluids involved. Cathodoluminescence indicates that sand injectites and fractures have one phase of cement associated with them. Seeps, however, show zoned carbonate cement, suggesting precipitation in a varying geochemical environment. This suggests that fractures and sand injectites were short-lived fluid-conduits, whereas seeps can act as fluid escape pathways over prolonged periods of time.

Fluid inclusion studies of chemosynthetic carbonates: strategy for seeking life on Mars.

Fluid inclusions in minerals hold the potential to provide important data on the chemistry of the ambient fluids during mineral precipitation. Especially interesting to astrobiologists are inclusions in low-temperature minerals that may have been precipitated in the presence of microorganisms. We demonstrate that it is possible to obtain data from inclusions in chemosynthetic carbonates that precipitated by the oxidation of organic carbon around methane-bearing seepages. Chemosynthetic carbonates have been identified as a target rock for astrobiological exploration. Other surficial rock types identified as targets for astrobiological exploration include hydrothermal deposits, speleothems, stromatolites, tufas, and evaporites, each of which can contain fluid inclusions. Fracture systems below impact craters would also contain precipitates of minerals with fluid inclusions. As fluid inclusions are sealed microchambers, they preserve fluids in regions where water is now absent, such as regions of the martian surface. Although most inclusions are < 5 microns, the possibility to obtain data from the fluids, including biosignatures and physical remains of life, underscores the advantages of technological advances in the study of fluid inclusions. The crushing of bulk samples could release inclusion waters for analysis, which could be undertaken in situ on Mars.

Fluid escape from reservoirs: implications fromcold seeps, fractures and injected sands Part II. The fluids involved

Petroleum fluids escape from hydrocarbon reservoirs through permeable networks of fractures, injected sands and by seepage through low permeability host rocks. Carbon stable isotope analysis of carbonate cement associated with such structures shows that the escaping petroleum fluid is intimately involved in precipitating carbonate cement. Within fractures and injected sands, oxidation of chained hydrocarbons supplies bicarbonate to the co-existing aqueous solution from which carbonate precipitates (y 13 C around 20xV-PDB). y 13 C values within carbonate crusts associated with seeps are lower (as low as 50xV-PDB) suggesting a component of carbon derived from methane within these structures. This suggests a separation within the escaping petroleum fluids, with chained hydrocarbons remaining trapped within sand injectites and fractures, whereas more buoyant fluids (methane) continue to escape through low permeability host rocks. The abundance of fluorescing (chained) hydrocarbon inclusions within cement associated with fractures and injected sands versus the scarce occurrence of fluorescing inclusions within carbonate crusts associated with seeps is in agreement with results from stable isotope analysis. Oxygen stable isotope ratios indicate that carbonate cement within fractures and sand injectites precipitates at temperatures between 30 and 50 jC in the subsurface, whereas carbonate cement associated with seeps at the seafloor precipitates at temperatures around 0 jC. D 2003 Elsevier Science B.V. All rights reserved.

The plumbing system feeding the Lusi eruption revealed by ambient noise tomography

Lusi is a sediment‐hosted hydrothermal system featuring clastic‐dominated geyser‐like eruption behavior in East Java, Indonesia. We use 10 months of ambient seismic noise cross correlations from 30 temporary seismic stations to obtain a 3‐D model of shear wave velocity anomalies beneath Lusi, the neighboring Arjuno‐Welirang volcanic complex, and the Watukosek fault system connecting the two. Our work reveals a hydrothermal plume, rooted at a minimum 6 km depth that reaches the surface at the Lusi site. Furthermore, the inversion shows that this vertical anomaly is connected to the adjacent volcanic complex through a narrow (~3 km wide) low velocity corridor slicing the survey area at a depth of ~4–6 km. The NE‐SW direction of this elongated zone matches the strike of the Watukosek fault system. Distinct magmatic chambers are also inferred below the active volcanoes. The large‐scale tomography features an exceptional example of a subsurface connection between a volcanic complex and a solitary erupting hydrothermal system hosted in a hydrocarbon‐rich back‐arc sedimentary basin. These results are consistent with a scenario where deep‐seated fluids (e.g., magmas and released hydrothermal fluids) flow along a region of enhanced transmissivity (i.e., the Watukosek fault system damage zone) from the volcanic arc toward the back arc basin where Lusi resides. The triggered metamorphic reactions occurring at depth in the organic‐rich sediments generated significant overpressure and fluid upwelling that is today released at the spectacular Lusi eruption site.

The Lusi seismic experiment: An initial study to understand the effect of seismic activity to Lusi

The spectacular Lumpur Sidoarjo (Lusi) eruption started in northeast Java on the 29 of May 2006 following a M6.3 earthquake striking the island [1,2]. Initially, several gas and mud eruption sites appeared along the reactivated strike-slip Watukosek fault system [3] and within weeks several villages were submerged by boiling mud. The most prominent eruption site was named Lusi. The Lusi seismic experiment is a project aims to begin a detailed study of seismicity around the Lusi area. In this initial phase we deploy 30 seismometers strategically distributed in the area around Lusi and along the Watukosek fault zone that stretches between Lusi and the Arjuno Welirang (AW) complex. The purpose of the initial monitoring is to conduct a preliminary seismic campaign aiming to identify the occurrence and the location of local seismic events in east Java particularly beneath Lusi.This network will locate small event that may not be captured by the existing BMKG network. It will be crucial to design the second pha...