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Bulletin Article

5726
 

Field analogs allow a better characterization of fracture networks to constrain naturally fractured reservoir models. In analogs, the origin, nature, geometry, and other attributes of fracture networks can be determined and can be related to the reservoir through the geodynamic history. In this article, we aim to determine the sedimentary and diagenetic controls on fracture patterns and the genetic correlation of fracture and diagenesis with tectonic and burial history. We targeted two outcrops of Barremian carbonates located on both limbs of the Nerthe anticline (southeastern France). We analyzed fracture patterns and rock facies as well as the tectonic, diagenetic, and burial history of both sites. Fracture patterns are determined from geometric, kinematic, and diagenetic criteria based on field and lab measurements. Fracture sequences are defined based on crosscutting and abutting relationships and compared with geodynamic history and subsidence curves. This analysis shows that fractures are organized in two close-to-perpendicular joint sets (i.e., mode I). Fracture average spacing is 50 cm (20 in.). Fracture size neither depends on fracture orientation nor is controlled by bed thickness. Neither mechanical stratigraphy nor fracture stratigraphy is observed at outcrop scale. Comparing fracture sequences and subsidence curves shows that fractures existed prior to folding and formed during early burial. Consequently, the Nerthe fold induced by the Pyrenean compression did not result in any new fracture initiation on the limbs of this fold. We assume that the studied Urgonian carbonates underwent early diagenesis, which conferred early brittle properties to the host rock.

5684
 
The fact that velocity models based on seismic reflection surveys commonly do not consider the near-surface geology necessitates filling the gap between the top of a velocity model and the surface of the Earth. In this study, we present a new workflow to build a shallow geologic model based exclusively on borehole data and corroborated by laboratory measurements. The study area is in Chemery (France), located at the southwestern border of the Paris Basin, where a large amount of borehole data is publicly available. The workflow starts with identifying lithologic interfaces in the boreholes and interpolating them between the boreholes. The three-dimensional (3-D) geometry of the lithologies then allows interpretation of the position, orientation, and offset of fault planes. Given the importance of the fault interpretation in the modeling process, a combination of different approaches is used to obtain the most reasonable structural framework. After creating a 3-D grid, the resulting 3-D structural model is populated with upscaled velocity logs from the boreholes, yielding the final near-surface P-wave velocity model. To better constrain the velocity model, we conducted laboratory measurements of P- and S-wave velocities in dry and water-saturated conditions on all lithologies in the model. The laboratory data were used to populate the 3-D near-surface model with VP/VS ratio values. The presented workflow accounts for one-dimensional borehole data and is much more iterative and time-consuming than workflows based on two-dimensional seismic sections. Nevertheless, the workflow results in a robust 3-D near-surface model allowing for structural interpretations and revealing the 3-D seismic velocity field.
5694
 
We describe the structure, microstructure, and petrophysical properties of fault rocks from two normal fault zones formed in low-porosity turbiditic arkosic sandstones, in deep diagenesis conditions similar to those of deeply buried reservoirs. These fault rocks are characterized by a foliated fabric and quartz-calcite sealed veins, which formation resulted from the combination of the (1) pressure solution of quartz, (2) intense fracturing sealed by quartz and calcite cements, and (3) neoformation of synkinematic white micas derived from the alteration of feldspars and chlorite. Fluid inclusion microthermometry in quartz and calcite cements demonstrates fault activity at temperatures of 195degC to 268degC. Permeability measurements on plugs oriented parallel with the principal axes of the finite strain ellipsoid show that the Y axis (parallel with the foliation and veins) is the direction of highest permeability in the foliated sandstone (10–2 md for Y against 10–3 md for X, Z, and the protolith, measured at a confining pressure of 20 bars). Microstructural observations document the localization of the preferential fluid path between the phyllosilicate particles forming the foliation. Hence, the direction of highest permeability in these fault rocks would be parallel with the fault and subhorizontal, that is, perpendicular to the slickenlines representing the local slip direction on the fault surface. We suggest that a similar relationship between kinematic markers and fault rock permeability anisotropy may be found in other fault zone types (reverse or strike-slip) affecting feldspar-rich lithologies in deep diagenesis conditions.
3693
 
Outcrops of the Cretaceous high-porosity sandstone of the Southeast Basin, France, show two main types of deformation structures: a large number of small-offset, shear-enhanced cataclastic deformation bands (DBs); and a small number of large (meters to decameters)-offset ultracataclastic fault zones. Microstructural analyses of the cataclastic DBs show that fragmentation produces strands of cataclastic fragment-supported matrix, separated by weakly fractured host rock, which cluster to form the DBs. The ultracataclastic fault zones, however, are composed of a matrix-supported ultracataclasite material. Permeability data show that the DBs reduce host-rock permeability by 0.5 to 2 orders of magnitude, whereas the ultracataclasites reduce permeability by approximately 4 orders. Simple calculations considering the structural frequency, thickness, and permeability of these faults suggest that, although the DBs may have an impact on single-phase flow, it is most likely to be less than a 50% reduction in flow rate in extensional contexts, but it may be more severe in the most extreme cases of structural density in tectonic shortening contexts. The larger ultracataclastic faults, however, despite their much lower frequency, will have a more significant reduction in flow rate, probably of approximately 90 to 95%. Hence, although they are commonly at or below the limit of seismic resolution, the detection and/or prediction of such ultracataclastic faults is likely to be more important for single-phase flow problems than DBs (although important two-phase questions remain). The study also suggests that it is inappropriate to use the petrophysical properties of core-scale DB structures as analogs to larger seismic-scale faults.

Bulletin Discussion

3704
 
We do not dispute that the pores shown in the photomicrograph of figure 8G of Beavington-Penney et al. (2008; reproduced here as Figure 1) could have formed at least partly by poststylolite dissolution, but we do not agree that this photomicrograph constitutes evidence for porosity creation by mesogenetic dissolution in the El Garia Formation of offshore Tunisia. Our skepticism is based on two main considerations: (1) that the multiple possible origins of the pores shown in Figure 1 cannot be determined with any meaningful degree of objective certainty and (2) that Figure 1 appears to be unrepresentative of pore types in the El Garia Formation, based on comparison with numerous other published images from this unit.

Explorer Article

1859
 
Explorer Article

Have an opinion about hydraulic fracturing? So does the person next to you, which is why Steve Leifer, this year’s speaker at the upcoming DEG annual luncheon, says to look at the big picture.  

2391
 
Explorer Article

AAPG’s prestigious Distinguished Lecturer program, which offers audiences a chance to hear the latest in geoscience research, understanding and practical applications, kicks-off its new season with three speaking tours planned for September.

Explorer Emphasis

2632
 
Explorer Emphasis Article

Seismic acquisition crews in southwest France attempt to gather data without disturbing the locals.

1861
 
Explorer Emphasis Article

An added dimension: 3-D seismic data is an essential ingredient in operators’ efforts to understand the risks and nuances of shale plays.

Explorer Regions and Sections

2199
 
Explorer Regions and Sections

For those of us in the AAPG European Region, 2011 began where 2010 left off – very busy.

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