Significance of large‐scale sand injectites as long‐term fluid conduits: evidence from seismic data

Abstract

Sand injectites and related features that are interpreted to have formed by large-scale, often sudden,¯uid escape in the shallow (typically <500 m) crust are readily imaged on modern seismic data. Many of the features have geometrical similarity to igneous dykes and sills and cross-cut the depositional stratigraphy. Sand injectites may be multiphase and form connected, high-permeability networks that transect kilometre-scale intervals of otherwise ®ne-grained, low-permeability strata. North Sea examples often form signi®cant hydrocarbon reservoirs and typically contain degraded, low-gravity crude oil. Fluid inclusion and stable isotope data from cements in sand injectites record a mixing of aqueous¯uids of deep and shallow origin.

Micro- to macro-scale internal structures, diagenesis and petrophysical evolution of injectite networks in the Vocontian Basin (France): Implications for fluid flow

“…This could be due to the effect of post-injection margin retractions on thin sand dykes, which tend to maintain higher fluid pressures and, subsequently, high porosities due to lateral compression. Alternatively, this relationship might illustrate that thin dykes become carbonate-cemented more quickly, thus preserving the granular framework from further porosity reduction by compaction (Jonk, 2003;Jonk et al, 2005a, b). A sill thickness > 0.5 m shows a positive correlation with the M A N U S C R I P T 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 29 29 minus-cement porosity, implying that porosities increase together with the widths of the injections.…”

“…Although injectites were first described in 1827 (Murchison), active research began over 150 years later when their undeniable roles in hydrocarbon systems were recognized (Dixon et al, 1995). In particular, injectites form vertical communications between reservoirs separated by low-permeability sedimentary intervals (Hurst et al, 2003). Large injectite networks also represent intrusive traps for fluids and excellent reservoirs containing a substantial volume of porous and permeable sand, forming excellent pay zones (Hurst and Catwright, 2007;Braccini et al, 2008).…”

Fluid flow features in hydrocarbon plumbing systems: What do they tell us about the basin evolution?

“…Subsequently, several observations of kilometer-scale sand injections have been made particularly in relation to Paleogene oil and gas fields of the North Sea (MacLeod et al, 1999;Duranti et al, 2002;Purvis et al, 2002;Hurst et al, 2003;Huuse et al, 2005). In seismic data the intrusions are recognized as high-amplitude reflections cross-cutting the surrounding strata giving rise to a variety of geometries including dykes, sills, conical, saucer-shaped and wing-like intrusions (Fig.…”

“…Several components or elements have however been recognized as constituting whole or parts of a fluid flow conduit including: gas chimneys, pipes, fractures including hydro-fractures, tectonic induced fractures and micro-fractures, irreversible deformations of the strata (e.g. bending, folding and faulting), diatremes, calderas, downward tapering cones, dikes and low-angled sills, leakage zones and hydrocarbon related diagenetic zones (HRDZ) (Brown, 1990;Cowley and O'Brien, 2000;Hurst et al, 2003;Stewart and Davies, 2006;Cartwright et al, 2007;Løseth et al, 2009). Apparent fluid flow conduits may in reality be seismic artifacts related to either migration anomalies, scattering artifacts, lateral velocity anomalies, or attenuation artifacts related to shallower high-amplitude features (e.g.…”

Using CSEM techniques to map the shallow section of seafloor: From the coastline to the edges of the continental slope

“…For example, deeper gas hydrate concentrations, trapped groundwater, channel sequences, and possibly sand injectites ͑e.g., Hurst et al, 2003͒. In principle, there is no reason why the system could not be extended to provide deeper penetration. The limit to this is probably a system of 400-500 m in length, comparable to the HED system operated by the University of Toronto ͑Yuan and Edwards, 2000;Schwalenberg et al, 2005͒.…”