Gregor P. Eberli

Controls on Sonic Velocity in Carbonates

Compressional and shear-wave velocities (Vp andVs) of 210 minicores of carbonates from different areas and ages were measured under variable confining and pore-fluid pressures. The lithologies of the samples range from unconsolidated carbonate mud to completely lithified limestones. The velocity measurements enable us to relate velocity variations in carbonates to factors such as mineralogy, porosity, pore types and density and to quantify the velocity effects of compaction and other diagenetic alterations.Pure carbonate rocks show, unlike siliciclastic or shaly sediments, little direct correlation between acoustic properties (Vp andVs) with age or burial depth of the sediments so that velocity inversions with increasing depth are common. Rather, sonic velocity in carbonates is controlled by the combined effect of depositional lithology and several post-depositional processes, such as cementation or dissolution, which results in fabrics specific to carbonates. These diagenetic fabrics can be directly correlated to the sonic velocity of the rocks.At 8 MPa effective pressureVp ranges from 1700 to 6500 m/s, andVs ranges from 800 to 3400 m/s. This range is mainly caused by variations in the amount and type of porosity and not by variations in mineralogy. In general, the measured velocities show a positive correlation with density and an inverse correlation with porosity, but departures from the general trends of correlation can be as high as 2500 m/s. These deviations can be explained by the occurrence of different pore types that form during specific diagenetic phases. Our data set further suggests that commonly used correlations like “Gardners Law” (Vp-density) or the “time-average-equation” (Vp-porosity) should be significantly modified towards higher velocities before being applied to carbonates.The velocity measurements of unconsolidated carbonate mud at different stages of experimental compaction show that the velocity increase due to compaction is lower than the observed velocity increase at decreasing porosities in natural rocks. This discrepancy shows that diagenetic changes that accompany compaction influence velocity more than solely compaction at increasing overburden pressure.The susceptibility of carbonates to diagenetic changes, that occur far more quickly than compaction, causes a special velocity distribution in carbonates and complicates velocity estimations. By assigning characteristic velocity patterns to the observed diagenetic processes, we are able to link sonic velocity to the diagenetic stage of the rock.

Factors controlling elastic properties in carbonate sediments and rocks

Carbonate sediments are prone to rapid and pervasive diagenetic alterations that change the mineralogy and pore structure within carbonate rocks. In particular, cementation and dissolution processes continuously modify the pore structure to create or destroy porosity. In extreme cases these modifications can completely change the mineralogy from aragonite/calcite to dolomite, or reverse the pore distribution whereby original grains are dissolved to produce pores as the original pore space is filled with cement to form the rock (Figure 1). All these modifications alter the elastic properties of the rock and, therefore, the sonic velocity. The result is a dynamic relationship among diagenesis, porosity, pore-type, and sonic velocity. The result is a wide range of sonic velocity in carbonates, in which compressional-wave velocity (VP) ranges from 1700 to 6600 m/s and shear-wave velocity (VS) from 600 to 3500 m/s.

Questioning carbonate diagenetic paradigms: evidence from the Neogene of the Bahamas

Carbonate diagenetic models have been heavily influenced by numerous studies of exposed Quaternary limestones. As a result, meteoric diagenesis is often assumed to be the principle means of altering aragonite-rich sediments into calcitic limestones. However, these models are limited by the scarcity of examples of aragonite-rich sediments buried in seawater that have never been influenced by meteoric fluids. The Bahamas transect cores recovered originally aragonite-rich sediments deposited in deep water beyond the easy reach of meteoric waters and provide an opportunity to test current diagenetic paradigms. The Bahamas transect consists of seven cores drilled in the prograding western margin of Great Bahama Bank. The two proximal cores (Clino and Unda) were drilled on the platform top and recovered shallow-water platform to reef facies overlying deeper margin and proximal slope facies. The five distal cores were drilled by ODP Leg 166 in up to 660 m of water and recovered carbonate slope facies. All studied sections are Neogene to Pleistocene in age. Diagenetic environments were identified based on petrographic and scanning electron microscopy (SEM) observations, XRD mineralogy, carbon and oxygen stable isotopic data, and trace elements. The upper 100^150 m of the two proximal cores were altered in meteoric to mixing-zone diagenetic environments but all other intervals were altered exclusively in marine pore fluids during seafloor, marineburial, and deep-burial diagenesis. Several of the findings of this study question current carbonate diagenetic paradigms. These include: (1) large-scale sea level lowstands may not have chemically active meteoric lenses as we found no meteoric alteration at the 3120 m elevation of the latest Pleistocene lowstand. Rather, phreatic meteoric diagenesis appears restricted to within W10 m of the land surface. (2) Mixing-zone diagenesis includes aragonite dissolution and minor LMC cementation but does not show the cavernous porosity or dolomitization predicted by mixing-zone diagenetic models. Current models are based on coastal mixing zones, which do not appear to be applicable to these more inland, and perhaps more typical, locations. (3) Marine-burial diagenesis produces a mature limestone with fabrics formerly considered diagnostic for meteoric diagenesis such as moldic porosity, aragonite neomorphism, blocky calcite spar and calcite microspar. However, oxygen stable isotopic data (average N 18 O=+ 1x) indicate alteration in marine pore fluids only. The character of marine-burial diagenesis is partially controlled by the nature of the sediment being altered. We have identified two end-member styles, an open-system style characterized by dissolution of aragonite without significant cementation and a more closed-system style with aragonite dissolution

Segmentation and coalescence of Cenozoic carbonate platforms, northwestern Great Bahama Bank

Seismic profiles over the northwestern Great Bahama Bank reveal that it is formed by the coalescence of three smaller platforms and significant lateral progradation. This conclusion is contrary to previous models which assumed that the bank is the result of continuous accumulation on one huge buildup. In Late Cretaceous to early Tertiary time, a north-south-trending depression, the Straits of Andros, separated an eastern platform, Andros bank, from a western platform, Bimini bank. Initially, the Straits of Andros had dimensions similar to the modem Tongue of the Ocean but was progressively filled from east to west In early(?) to middle Tertiary time, a second depression, the Bimini embayment, formed within Bimini bank by folding; it had a maximum depth of about 470 m. This depression was also filled from east to west. In addition, prograding systems built the western margin of Bimini bank more than 25 km westward into the Straits of Florida.

Quantification of pore structure and its effect on sonic velocity and permeability in carbonates

Carbonate rocks commonly contain a variety of pore types that can vary in size over several orders of magnitude. Traditional pore-type classifications describe these pore structures but are inadequate for correlations to the rocks physical properties. We introduce a digital image analysis (DIA) method that produces quantitative pore-space parameters, which can be linked to physical properties in carbonates, in particular sonic velocity and permeability. The DIA parameters, derived from thin sections, capture two-dimensional pore size (DomSize), roundness (), aspect ratio (AR), and pore network complexity (PoA). Comparing these DIA parameters to porosity, permeability, and P-wave velocity shows that, in addition to porosity, the combined effect of microporosity, the pore network complexity, and pore size of the macropores is most influential for the acoustic behavior. Combining these parameters with porosity improves the coefficient of determination (R2) velocity estimates from 0.542 to 0.840. The analysis shows that samples with large simple pores and a small amount of microporosity display higher acoustic velocity at a given porosity than samples with small, complicated pores. Estimates of permeability from porosity alone are very ineffective (R2 = 0.143) but can be improved when pore geometry information PoA (R2 = 0.415) and DomSize (R2 = 0.383) are incorporated. Furthermore, results from the correlation of DIA parameters to acoustic data reveal that (1) intergrain and/or intercrystalline and separate-vug porosity cannot always be separated using sonic logs, (2) P-wave velocity is not solely controlled by the percentage of spherical porosity, and (3) quantitative pore geometry characteristics can be estimated from acoustic data and used to improve permeability estimates.

Extensional detachment faulting in the evolution of a Tethys passive continental margin, Eastern Alps, Switzerland

The Austroalpine nappes of Switzerland represent an exhumed and tectonically imbricated segment of the passive continental margin of the Jurassic Tethys. Within one of these nappes (Err nappe), part of an upper-crustal, extensional detachment is preserved, indicating that the thinning of the crust was achieved by non-uniform extension. The Mesozoic age of the detachment is shown by comparison between its associated cataclasites and identical cataclasites that are found as redeposited components in Middle(?) Jurassic sedimentary breccias. This low-angle detachment within the basement was kinematically linked to synsedimentary high-angle normal faults at the surface. The area of the Err nappe belongs, in terms of Jurassic paleogeography, to the most distal part of the continental margin, where both low- and high-angle normal faults dipped oceanward (that is, west to northwest in present-day coordinates). In the more proximal part of the margin, however, the high-angle normal faults dipped eastward toward the continent. Ammonite stratigraphy within the sediment prisms adjacent to the faults gives evidence for an Early Jurassic age of the faulting in the proximal part of the margin, whereas in the distal part, faulting occurred during latest Early to Middle Jurassic time. We therefore propose that the Jurassic extension of the crust, which finally led to the opening of the Piemont-Ligurian ocean, was achieved by two fault systems, which differ in geometry, fault orientation, and age. These fault systems were composed of a basal low-angle detachment with high-angle normal faults above. The orientation of the older, eastward-dipping detachment was prone for reactivation during early Alpine crustal shortening. The present-day thrust contact between the two major tectonic units of the area, the Lower Austroalpine and the Central Austroalpine nappe complex, therefore might correspond to an eastward-dipping, Jurassic low-angle normal fault.

The Velocity-Deviation Log: A Tool to Predict Pore Type and Permeability Trends in Carbonate Drill Holes from Sonic and Porosity or Density Logs

The velocity-deviation log, which is calculated by combining the sonic log with the neutron-porosity or density log, provides a tool to obtain downhole information on the predominant pore type in carbonates. The log can be used to trace the downhole distribution of diagenetic processes and to estimate trends in permeability. Laboratory measurements on over 300 discrete carbonate samples reveal that sonic velocity is a function not only of total porosity, but also of the predominant pore type. In general, there is an inverse porosity-velocity correlation, but significant deviations occur from this relationship for certain pore types. Frame-forming pore types, such as moldic or intrafossil porosity, result in significantly higher velocity values at equal total porosities than do pore types that are not embedded in a rigid rock frame, such as interparticle porosity or microporosity. The results of the laboratory measurements can be applied to expand interpretations of standard wireline-log data, as shown in this study on two drill holes through Neogene carbonates from the Great Bahama Bank. The velocity-deviation log is calculated by first converting porosity-log data to a synthetic velocity log using a time-average equation. The difference between the real sonic log and the synthetic sonic log can then be plotted as a velocity-deviation log. Because deviations are the result of the variability of velocity at a certain porosity, the deviation log reflects the different rock-physical signatures of the different pore types. Positive velocity deviations mark zones where velocity is higher than expected from the porosity values, such as zones where frame-forming pore types dominate. Zero deviations show intervals where the rock lacks a rigid frame, such as in carbonates with high interparticle porosity or microporosity. Negative deviations mark zones in which sonic log velocities are unusually low, caused, for instance, by a cavernous bore-hole wall, fracturing, or possibly by a high content of free gas. By tracing the velocity deviations continuously downhole, one can identify diagenetic zones that are characterized by these different pore types. In addition, this method can be used to observe permeability trends because pore types influence the permeability of the rock.

Microbial carbonates as indicators of environmental change and biotic crises in carbonate systems: examples from the Late Devonian, Alberta basin, Canada

Microbial precipitation of calcium carbonate has played a vital role in the development of carbonate platforms since their initiation in the Proterozoic. We report here the varied roles that microbial carbonates played in Late Devonian carbonate platforms in the Alberta basin, Canada. We recognize microbial carbonates as important contributors within the carbonate system during times of major environmental change including transgressive events in platform environments and the recovery interval following the Frasnian^Famennian mass extinction. Detailed sequence stratigraphic analysis of two isolated platforms in the Canadian Rockies was used to document their evolution from a regional ramp to isolated platforms with phases of progradation, aggradation and backstepping, and renewed progradation related to rates of second-order and third-order sea-level change and basin infill. The carbonate system was reorganized following annihilation of many carbonate-producing biota during the Frasnian^Famennian mass extinction. Microbial carbonates figure prominently in both Frasnian platform development and the Famennian recovery of the carbonate system following the Frasnian^Famennian mass extinction. During transgressive platform phases deeper-water facies with a crinoid, rugose coral, and microbe dominated biota abruptly overlie prograding stromatoporoid framestones. Microbial carbonates consist of abundant Girvinellid oncoids that nucleated around other fossil clasts. The temporary replacement of stromatoporoid communities during transgression implies a shift from oligotrophic to mesotrophic conditions and the microbes likely capitalized on the change in nutrient supply. Microbes played a significant role in highstand reef margin facies where Renalcis was a binder and cementer in stromatoporoid framestones and downslope rugose coral/stromatoporoid mounds. Microbially laminated carbonates also form stromatolitic mats in peritidal shallowing-upward cycles and repetitive stromatolitic intervals commonly indicate sea-level lowstands associated with the development of sequence boundaries. Stromatoporoid reefs and their associated reef interior facies indicate an oligotrophic ecosystem where microbial carbonates were relegated mainly to cryptic or stressed marine habitats. An end-Frasnian sea-level lowstand exposed the isolated carbonate platforms. Transgression and mass extinction characterize the Frasnian^Famennian boundary event and microbial carbonates occur at this horizon in western Canada. Stromatolites, oncoids, and large-scale microbial thrombolites appear to be

Porosity-permeability relationships in interlayered limestone-dolostone reservoirs

Porosity and permeability data from five carbonate platform successions of different settings, ages, and burial depths are examined to identify overall similarities and differences in the reservoir quality of interlayered limestones and dolostones. Each succession consists mainly of limestone and dolostone, with subordinate proportions of intermediate, partly dolomitized compositions. In the three deeply buried platforms, the key features are that limestones have much lower average porosity than associated dolostones, and that limestones and dolostones show little difference in average permeability-for-given-porosity. In contrast, the shallowly buried platforms show little difference in average porosity between limestones and dolostones and also display higher average permeability-for-given-porosity in dolostones than limestones. These data suggest the following general guidelines for depositional and diagenetic controls on reservoir architecture in carbonates consisting of interlayered limestone and dolostone. (1) Reservoir compartmentalization by the formation of tight limestone barriers is largely a burial diagenetic process involving calcite cementation locally produced by chemical compaction. (2) Both the pattern of early dolomitization and the distribution of clay minerals (because they influence the localization of chemical compaction) are key factors that determine the distribution of tight limestone barriers separating flow units. Thus, the pattern of eventual burial compartmentalization follows a template that is hardwired into the stratigraphic architecture by depositional mineralogy and early diagenesis. (3) After burial (2–3 km; 1.2–1.8 mi), dolostones should not be expected to have higher permeability-for-given- porosity than associated limestones. These rules assume dolomitization to occur early relative to chemical compaction, which will commonly be the case because of combined hydrologic and mass-balance constraints. Dolomitization and concentration of clay appear linked to cycle and sequence architecture in the present examples and thus may have a useful degree of predictability, at least in terms of statistical parameters, such as net/gross and probability of flow-unit thickness distribution.

Changes of shear moduli in carbonate rocks: Implications for Gassmann applicability

In laboratory experiments we measured the saturation effects on the acoustic properties in carbonates and the results question some theoretical assumptions. In particular, these laboratory experiments under dry and wet conditions show that shear moduli do not remain constant during saturation. This change in shear modulus puts Gassmanns assumption of a constant shear modulus into question and also explains why velocities predicted with the Gassmann equation can be lower or higher than measured velocities.