The Determination of transport properties in carbonate rocks with high-resolution CT and NMR.

Continental carbonates are studied because of their reservoir potential. Especially with the recent discovery of microbialite and travertine oil fields in the Pre-Salt (offshore Brazil) and in the Namibe basin (Angola), the importance of continental carbonates as reservoir rocks for future hydrocarbon production was highlighted. Quaternary continental carbonates from quarries in Denizli (Turkey) and from Sütt# and Budakalász (Hungary) were therefore studied as reservoir analogues in this PhD study.

Continental carbonates are well known to be anisotropic and heterogeneous. These rocks not only contain complex primary fabrics, they are also often modified by diagenesis, resulting in the most complex pore networks in natural rock samples. Continental carbonate reservoirs present therefore some of the greatest challenges to develop or refine technologies and processes that maximize their hydrocarbon recovery. Due to the lack of straightforward porosity and permeability relationships, there is a strong need for a clear understanding of the pore network in continental carbonates, which will help to interpret seismic data and to build improved conceptual reservoir models.

This study contributes to the understanding of pore networks in continental carbonates, by applying an integrated approach based on lab porosity and permeability measurements, geostatistics, Computer Tomography (CT), Nuclear Magnetic Resonance (NMR), Mercury Injection Porosimetry (MIP), permeability simulations and acoustic velocity measurements. The integration of these techniques allows characterizing pore network properties from micro- to macroscale and to unravel their influence on petrophysical characteristics in representative elementary volumes.

Four main continental carbonate lithofacies, i.e. sub-horizontal, reed, cascade and waterfall facies were identified in the studied quarries. Within these facies 15 main lithotypes and 7 dominant pore types were described. Peloidal, phyto and dendritic lithotypes dominate the continental carbonates samples. Frequently occurring pore types are microporosity, interpeloidal porosity, interlaminar porosity, moldic porosity, micro-moldic porosity, vuggy porosity and framework porosity.

The studied continental carbonates exhibit large-scale ranges for both porosity (from a few percent to over forty percent porosity) and permeability (between 0.001mD and several tens of Darcys. The heterogeneity results from complex primary fabrics, textures, pore type variability and diagenetic alterations. Given their heterogeneity, geostatistical analyses allowed to better demonstrate the control of facies types on porosity and permeability. The sub-horizontal facies for example yields the lowest porosities and permeabilities, the waterfall facies the highest. Geostatistics furthermore provided an upscaling from discrete measurements of logged sections to a full 3D porosity, permeability and facies distribution model on reservoir scale.

Continental carbonate samples were subdivided into three categories based on the MIP and NMR pore size distributions. The pore network behavior on sub-facies scale relates to the distribution type and their dominant lithotypes. Samples from the sub-horizontal and cascade facies are very often unimodal and dominated by the peloidal lithotype. Peloids are expected to result from the calcification of bacterial matter or from microdetrital allochtonous processes. Samples from the reed and waterfall facies are mainly from the bimodal and atypical type, resulting from their phyto lithotype. The main difference with samples dominated by peloidal lithotypes is the high occurrence of plant molds. In order for plants to grow, the environment should not be too harsh and the water should not be too deep or too hot. The bimodal samples are typically highly porous, but their permeability is lower compared to samples of the atypical type. The decoupled micropore compartments in bimodal samples increase the effective porosity, but will not contribute to pore connectivity. Atypical samples contain uncemented pores and coupled micropores, which provide better fluid pathways and the highest permeabilities. Micropores in continental carbonates can thus influence the connectivity and fluid flow in the samples.

Simulated permeabilities on reconstructed pore networks correspond well with physical core permeability measurements. Porosity and tortuosity govern the fluid flow in continental carbonates and integrating both parameters is necessary to predict the fluid flow through the complex pore network of continental carbonates. The spatial resolution of the CT scans affects the simulated permeabilities. Multi Point geo-Statistics (MPS) allow modeling large artificial rock volumes at higher resolutions. Simulated permeabilities in the rock models are in the same order of magnitude as physical permeability measurements and demonstrate similar facies and pore type dependencies as observed in natural continental carbonates.

Acoustic velocities in continental carbonates show a first-order dependency on porosity, with an inverse linear relation. Second-order velocity deviations correlate with pore size and shape complexity. Small and complex pores are associated with negative acoustic velocity deviations. Large, simple and stiff pores result in increased velocities. The velocities increase even further with cementation. Velocities in vuggy continental carbonates are more difficult to predict. The variable pore shapes and sizes in the latter can accelerate or slow down wave propagation. Acoustic impedances in continental carbonates generate seismic reflectors in seismic sections of mono-mineralic carbonate systems. The reflectors are not caused by non-carbonate intercalations, but relate to geobody boundaries, in which the seismic expression is function of porosity and pore types.

Vp/Vs ratios for the continental carbonates fell between 1.8 and 2 for the entire Vp domain. Such ratios are typical for indurated carbonates and are indicative for the compressive strength of the rock frame. The Vp/Vs ratio of continental carbonates can partly explain the porosity preservation that was reported in literature for Pre-Salt continental carbonate reservoirs. The Pre-Salt continental carbonate framestones, with Vp/Vs ratios that are expected to be similar, can likely also preserve part of their primary porosity, due to the rigidity of the rock frame. Other processes that could have created or reduced porosity or could have had a frame stabilizing effect in the Pre-Salt continental carbonates are dissolution, dolomitisation and cementation, with for example carbonate or siliceous cements. Continental carbonates are a crystalline precipitate, in contrast to platform carbonates, with which they were compared in this study. The platform carbonates formed mostly by the accumulation of skeletal fragments, as well as carbonate secreting animals and plants, resulting in acoustic velocities that largely follow the reference trendline for limestone in the compressional-wave velocity versus porosity cross plot. Acoustic velocities in continental carbonates differ from porosity – velocity transforms in marine platform carbonates. Continental carbonates, plotting well above the limestone reference line, produce acoustic impedances that can possibly distinguish the latter from marine platform carbonates in seismic sections. Biologically-induced or biotically controlled marine carbonates, such as mud-mounds and reefal buildups could show more similarities with the here investigated continental carbonates. The latter mound structures can form micro- to macroporous rigid framework reefs, with a different lithology, formed by carbonate mud, peloidal mud, micrite and in situ skeletal metazoans. Conceptually, the rigid framework reefs could thus produce acoustic velocities that lean more towards those encountered in continental carbonates.

The multi-technique methodology proved to be applicable in other carbonate lithologies, e.g. dolomite and chalk and demonstrated the same potential in unraveling pore network properties from micro- to macroscale.

The unique 3D reconstruction of the Denizli carbonate dome (Turkey) can potentially serve as an analogue for sub-surface, offshore continental carbonate geobodies, as for example the domal carbonate build-ups in the Pre-Salt of the Atlantic Ocean. With the addition of samples affected by dissolution and cementation from the Sütt# and Budakalász quarries (Hungary), a broader spectrum of continental carbonate samples was covered. With the presented multi-technique approach, the study unravels the role of different pore types, pore sizes, litho- and facies types in the pore network and provides the best possible comprehension of the pore network behavior in complex continental carbonate samples.