Physical and mechanical properties of carbonate sedimentary systems

Reijmer, J.J.G. [Promotor]Bertotti, G.V. [Copromotor

Acoustic properties in travertines and their relation to porosity and pore types

Sonic velocities of Pleistocene travertines were measured under variable confining pressures. Combined with petrographical characteristics and petrophysical data, i.e. porosity, permeability and density, it was determined that travertine porosity, pore types and cementation control compressional-wave (Vp) and shear-wave velocity (Vs). At 40 MPa confining pressures, Vp ranges between 3695 and 6097 m/s and Vs between 2037 and 3140 m/s. Velocity variations in travertines are, as with all carbonates, primarily linked to sample heterogeneity, i.e. differences in fabric, texture and porosity. They thus not necessarily emanate from changes in mineralogy or composition. Body wave velocities have a positive correlation with sample density and an inverse correlation with porosity. The travertines, sampled in extensional settings with normal faulting activity, define a specific compressional-wave velocity (y-axis) versus porosity (x-axis) equation, i.e. (log(y) = −0.0048x + 3.7844) that differs from the Vp-porosity paths defined by marine carbonates. Acoustic wave velocities are higher for travertines than for marine carbonates. Travertine precipitates form rigid rock frames, often called framestone, with large primary pores. Marine carbonates on the other hand often consist of (cemented) transported sediments, resulting in a rock frame that permits slower wave propagation when compared to the continental limestones.Acoustic velocity variations are linked to variations in pore types. Mouldic pores (macropores) show faster wave propagation than expected from their total porosities. Microporosity, interlaminar and interpeloidal porosity result in slower acoustic velocities. Framework pores and micro-moulds are associated with lowered acoustic velocities, while vug porosity is found above, on and below the general velocity-porosity trend. Not only the pore type, but also pore shapes exert control on body wave velocities. Cuboid-and rod-like pore shapes increase the velocity, while plate-and blade-like pore shapes have a negative effect on the velocity. The study demonstrates how seismic sections in travertine systems can contain seismic reflections that are not caused by non-carbonate intercalations, but relate to geobody boundaries, in which the seismic expression is function of porosity, pore types and shapes. This study provides and relates petrophysical data, i.e. porosity, permeability and acoustic velocities of travertines and is of importance for the interpretation of seismic reflection data in subsurface continental carbonate reservoirs

Variations in petrophysical properties along a mixed siliciclastic carbonate ramp (Upper Jurassic, Ricla, NE Spain)

A multi-scale petrophysical study was performed on an Upper Jurassic siliciclastic - carbonate ramp system near the village of Ricla (Spain). A combination of textural (field observations, thin section and grain-size analysis), chemical - mineralogical (thermo gravimetric analysis) and physical (porosity, gamma ray and velocity measurements) properties was used to capture the heterogeneity of this mixed sedimentary system. Subsequently, synthetic seismic sections with varying resolution were created to show the characteristics of the internal ramp-structures. The translation from the measured petrophysical properties to a synthetic seismic section helps to better understand subsurface seismic. Although upscaling problems are a recurring issue, the homogeneity of the samples and the continuity of the outcrop and facies reduce the number of uncertainties.Acoustic properties are generally controlled by the amount and type of pore spaces. Due to the low porosities (φ < 7.2%) of this mixed ramp system the influence on the acoustic properties is limited; therefore other controlling parameters come into play. One of the primary controlling factors is the mineral composition, thus the presence of clay and the ratio between siliciclastic and carbonate components. A dominant presence of carbonate relates to relative high P- and lower S-wave velocities, a high percentage of quartz relates to relative lower P- and higher S-wave velocities, the presence of clay reduces the overall velocities even more. Carbonate infill of intergranular space (micrite or cement) slightly influences the acoustic properties. In general, the infill of microporous micrite shows lower velocities compared to the cemented samples. Finally microcracks may significantly reduce the velocity, although these cannot be quantified, their presence can be estimated using elastic property models.The ramp geometry is characterized by an overall lateral homogeneity and vertical heterogeneity. Textural, sedimentological and stratigraphic transitions in the outcrop are related to the defined facies, each has its own unique mineralogical signature. Amplitude variations in the synthetic seismic profile are primarily caused by changes in mineral composition thus facies transitions

Seismic characterization of switching platform geometries and dominant carbonate producers (Miocene, Las Negras, Spain)

The primary goals of seismic interpretation and quantification are to understand and define reservoir architecture and the distribution of petrophysical properties. Since seismic interpretation is associated with major uncertainties, outcrop analogues are used to support and improve the resulting conceptual models. In this study, the Miocene carbonates of Cerro de la Molata (Las Negras, south-east Spain) have been selected as an outcrop analogue. The heterogeneous carbonate rocks of the Cerro de la Molata Platform were formed by a variety of carbonate-producing factories, resulting in various platform morphologies and a wide range of physical properties. Based on textural (thin sections) and petrophysical (porosity, density, carbonate content and acoustic properties) analyses of the sediments, eleven individual facies types were determined. The data were used to produce synthetic seismic profiles of the outcrop. The profiles demonstrate that the spatial distribution of the facies and the linked petrophysical properties are of key importance in the appearance of the synthetic seismic sections. They reveal that carbonate factory and facies-specific reflection patterns are determined by porosity contrasts, diagenetic modifications and the input of non-carbonate sediment. The reflectors of the seismograms created with high-frequency wavelets are coherent with the spatial distribution of the predefined facies within the depositional sequences. The synthetic seismograms resulting from convolution with lower frequency wavelets do not show these details – the major reflectors coincide with: (i) the boundary between the volcanic basement and the overlying carbonates; (ii) the platform geometries related to changes in carbonate factories, thus sequence boundaries; and (iii) diagenetic zones. Changes in seismic response related to diagenesis, switching carbonate producers and linked platform geometries are important findings that need to be considered when interpreting seismic data set

Variations in petrophysical properties of Upper Palaeozoic mixed carbonate and non-carbonate deposits, Spitsbergen, Svalbard Archipelago

The Late Carboniferous – Early Permian Gipsdalen Group and the Early to Late Permian Templefjorden Group are known hydrocarbon plays in the Arctic region, e.g. on the Finnmark Platform, Loppa High and Sverdrup Basin. Time-equivalent deposits crop out on the island of Spitsbergen and consist of mixed carbonate and non-carbonate (primarily siliciclastic, siliceous, organic-carbon rich and clayey) sediments deposited in continental to deep-marine settings. In rock samples (n = 73) collected from five outcrop locations on Spitsbergen, thin-section analysis showed the presence of ten microfacies types ranging from claystones and spiculitic cherts to rudstones and dolostones. Petrophysical and textural properties of the samples were measured to evaluate the link with the acoustic (P- and S-wave) velocities of these generally tight rocks, which have an average porosity of about 2%. Variations in acoustic velocity measurements primarily depend on variations in mineralogical composition (silica versus carbonate) and, to a lesser extent, on variations in porosity and bulk density. Pore networks in the sediments are dominated by microporosity and (micro)cracks, followed by interparticle porosity. Recrystallization effects and pore shape variations show a lesser effect on the P-wave velocity. Clay content does not exceed 12.7% and also has a secondary impact on the acoustic velocities. Defining which textural and physical parameters control the acoustic properties of these carbonate and non-carbonate sedimentary rocks will help with the interpretation of the seismic response of equivalent deposits in the subsurface

Fracture distribution along an Upper Jurassic carbonate ramp, NE Spain

Understanding the distribution of natural fractures in sedimentary systems is of high relevance for the exploration and production of fluids in the subsurface. This study focuses on a Kimmeridgian mixed siliciclastic-carbonate ramp system, which is part of the Jurassic limestones outcropping in the northeastern Iberian Chain. The study area is located north of the Ricla village, fifty kilometers southwest of Zaragoza. The outcrop stretches over six kilometers in length, it allows for recording detailed fracture patterns and facies variations. A GIS-based software-package 'DigiFract' is used to measure and digitize fractures in vertical outcrops. Fracture orientation measurements obtained from vertical and horizontal outcrops are used to create a conceptual three-dimensional image of the fracture distribution of the mixed ramp system. The fracture data are processed by integrating outcrop logs and sample-data obtained from thin-sections and rock property analysis. The continuous facies belts of the shallow low-angle ramp system show limited lateral variations. The studied sediments are subdivided in four main facies tracts (FT's): (FT-1) Bioclastic siltstones, (FT-2) alternating sandy limestones and marls, (FT-3) cross-bedded and channelled oolitic-bioclastic sand- and grainstones and (FT-4) coralgal float-to mudstones with eventites. Measured fracture orientations of the vertical and horizontal outcrops are identical throughout the entire exposure. Two main fracture sets are identified, the first set (Set 1) has a N-S direction and the second set (Set 2) has a NE-SW direction. The lateral homogeneity of the sedimentary system, thus facies, layer thickness and slope angle, can be translated to the observed fracture patterns. Within one single facies belt, fractures tend to behave the same in the proximal, middle and distal part of the ramp system. However, vertical facies variations are an important factor for the measured vertical fracture-heterogeneity. Fine-grained mud-supported facies correspond to periods of a sea-level highstand; coarse-grained cemented facies on the other hand are related to a sea-level lowstand. The physical contrast of the sediments caused by sea-level fluctuations forces fractures to solely concentrate in the brittle layers. Fracture density and termination patterns observed on this ramp stand in contrast to fracture geometries observed in flat-topped carbonate platforms. Lateral facies heterogeneity and platform anatomy of flat-topped carbonate platforms are key parameters for the eventual fracture distribution. For the studied mixed ramp-type system the vertical facies variations are key parameter

Pore space evolution and elastic properties of platform carbonates (Urgonian Limestone, Barremian-Aptian, SE France)

A dataset of 214 ultrasonic velocity and porosity measurements on Barremian-Aptian carbonates from Provence (SE France) provides well-constrained velocity-porosity transforms and allows the quantification of the impact of pore type and diagenetic history on these velocities. A numerical approach (EPAR: equivalent pore aspect ratio) was used to link diagenetic transformations, pore network evolution and elastic properties. Three categories of samples characterized by their dominant pore type were discriminated from the velocity and porosity database by means of the EPAR values derived from bulk (K-EPAR) and shear (μ-EPAR) moduli: 1) purely microporous limestones (low K- and μ-EPAR . 0.3). Three velocity-porosity trajectories related to three diagenetic paths were defined and quantified from the Urgonian database: 1) EPAR-preserving micro-scale cementation of micrite, 2) non-EPAR-preserving dissolution with moldic pore development and 3) EPAR-preserving sparry calcite cementation of molds. Equivalent Pore Aspect Ratio can therefore be regarded as a robust tool to decipher diagenetic trends in velocity-porosity transforms and may help predicting pore architecture from subsurface data. © 2014 Elsevier B.V