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Fluid flow due to the advance of basin-scale silica reaction zones

Richard J. Davies^,1, Neil R. Goulty¹ and David Meadows¹

¹ CeREES (Centre for Research into Earth Energy Systems), Department of Earth Sciences, Durham University, Science Laboratories, South Road, Durham, DH1 3LE, UK

View larger version (99K): [in this window] [in a new window]   Figure 1. (A) Siliceous successions before and after passing through an opal-A to opal-CT reaction zone. Given no change in the thermal regime or sediment composition, the depth of conversion (DOC) remains fixed, which, in this example, is taken to be 500 m below the seabed. As the succession undergoes burial, the reaction zone "advances" upward through the sediment pile (alternatively, one could view it as sediment subsiding through the reaction zone). If the conversion causes a reduction in sediment porosity from 70% to 45%, then 500 m of stratigraphy (marked T1) will measure 250 m in thickness (T2) after transformation, and new accommodation space would be created and filled. Given continuous burial, the reaction zone will keep advancing through the section. Porosity-depth curves usually show undercompaction for the biosiliceous succession prior to conversion with a sharp kink in the porosity-depth curve across the reaction zone. Inset SEM (scanning electron microscope) photographs show the typical composition of sediment before and after conversion. (B) Typical two-dimensional seismic line from the Faeroe-Shetland Channel showing opal-A to opal-CT reaction zone (drilled by well 214/4-1). The reaction zone is a bright (high-amplitude), continuous reflection, with the same polarity as the seabed and cross cuts stratigraphy.

View larger version (142K): [in this window] [in a new window]   Figure 2. (A) Map of the Faeroe-Shetland channel (bathy metric contours in meters). Black rectangle shows the location of part B. (B) Colored contour map made for the reflection marked in green on the seismic line in C, which is an Early Pliocene unconformity. Contours are in meters: blue denotes lows; yellow and pink denote highs. (C) Seismic line X-Y showing an erosional crater, 500 m wide, 1000 m in length, and 100 m deep, at the Early Pliocene unconformity. The crater is linked downward to the opal-A to opal-CT reaction zone by a region of disrupted reflections. T—Truncated reflections. Blue—reaction zone, black—intermediate reflections, green—Early Pliocene unconformity (interpreted to be the contemporaneous seabed at the time of the fluid flux).

View larger version (114K): [in this window] [in a new window]   Figure 3. (A) Map of the Faeroe-Shetland channel (bathymetric contours in meters). Black rectangle shows the location of part B. (B) Dip map of reflection 300 m above the reaction zone (marked in red in C). Dark gray denotes dips >30°; light gray denotes dips <30°. Changes in dip reveal faults and hummocks described by Davies et al. (1999) and Davies (2005). Faults are marked "F". (C) Seismic-data volume with the overburden removed to reveal the Early Pliocene unconformity horizon. A fault (marked F) cuts the reaction zone and reaches the Early Pliocene unconformity where a series of high-amplitude, roughly circular features (marked X) track the blue fault trace.

View larger version (94K): [in this window] [in a new window]   Figure 4. Seismic line from a two-dimensional seismic survey from offshore Sakhalin island, showing an opal-A to opal-CT reaction zone with vertically nested depressions emanating from just above the level of the reaction zone. The vertical features are candidate fluid-escape structures. Black box marks concordant, continuous seismic reflections. Inset: location map showing Sea of Japan (SOJ) and North Sakhalin Island (SI), eastern Russia. Short red line on the inset map marks the location of the seismic line.

View larger version (7K): [in this window] [in a new window]   Figure 5. Porosity-depth graphs for siliceous sediments from: (A) Point Pedernales, Monterey Formation, California (Compton, 1991); and (B) New Jersey Coastal Plain, offshore east coast North America (Ocean drilling Project [ODP] Leg 150, Site 903) (Shipboard Scientific Party, 1994). Dashed line indicates the approximate position of the opal-A to opal-CT transition.

View larger version (61K): [in this window] [in a new window]   Figure 6. Intersection of commercial borehole 214/4-1 and a representative seismic line from the three-dimensional seismic survey. The opal-A to opal-CT transition zone is coincident with a bed or package of beds with high opal-CT content, and a reflection event corresponding to this lithological contrast can be mapped on the three-dimensional seismic data set (solid blue line). The anomalously high seismic amplitude at the reaction zone indicates an increase in acoustic impedance. Other reflections that have anomalously high seismic amplitude above the reaction zone probably represent sediment with a high percentage of opal-A that has started to transform at a shallower (DOC) (marked DOC 1 and with a green line).

View larger version (10K): [in this window] [in a new window]   Figure 7. Schematic graphs of reaction zones. (A) The reaction zone can be considered graphically as a curve of the number of moles being converted per unit time against depth below the depth of conversion (DOC). The rate of conversion at the depth of conversion (DOC) is zero, increases to a maximum, and then trails off to zero where the reaction is complete. In reality, in interbedded successions where the clay concentration changes from one bed to the next, the reaction zone should be highly complex with increases and decreases with depth in the rate of conversion of opal-A to opal-CT. The reaction zone advances through the succession as it undergoes burial from its position at time 1 to its position at time 2 and time 3. (B) Schematic graph of the number of moles of opal-A being converted per unit time against depth for two different reaction zones, one wide (W1) and one narrow (W2). The rate of fluid production per unit volume is higher with a narrow reaction boundary than with a wide reaction boundary, but the total rate of fluid production at each front is the same, because fluid is produced across the entire reaction zone.

View larger version (82K): [in this window] [in a new window]   Figure 8. (A) Map covering the Faeroe-Shetland, Møre Basins, and Vøring Basin (marked FSB, MB, and VB, respectively), as well as locations for Figure 2. (B) Example of an expanse of opal-A to opal-CT reaction boundary from the Møre Basin. The total coverage of the pattern on three-dimensional seismic data is 1000 km2, a subset of which is presented in this figure. The reaction zone is dominated by the development of a cell-like pattern. Cells are separated by a network of trough-like depressions. This is one of many geometries the reaction zone can adopt.