Diagenetically Controlled Elastic Properties and Seismic Expression of Carbonate Reservoirs (South-Eastern Basin, Provence, France)

R. Toullec¹, F. Fournier¹, P. Leonide¹, L. Kleipool², J. Reijmer², J. Borgomano¹, and C. E. Harris³

¹AIX-Marseille University, CEREGE, Marseille, France
²VU University Amsterdam, Department of Sedimentology and Marine Geology, Amsterdam, The Netherlands
³ExxonMobil Upstream Research Company, Houston, Texas, USA

Carbonate reservoirs are essentially heterogeneous complex media that form in various depositional environments and are overprinted by diagenetic reactions at various times in their history. The refinement of seismic acquisition and processing can reveal this complexity through specific reflection patterns observable on seismic lines.

The objectives of this study are to assess the impact of diagenesis on reservoir and elastic properties of carbonates at all scales and to evaluate this impact on their seismic expression. To pursue these objectives, two seismic-scale carbonate reservoir analogues were chosen in the Southeastern Basin in France : i) The early stratiform dolomites and fault-related dolomite bodies from Upper Jurassic of “Massif de l'Etoile” and “Nerthe” areas, analogous of Khuff reservoirs ii) The microporous limestones from lower Cretaceous of “Mt de Vaucluse”, analogous of Kharaib/Shuaba reservoirs (presented here). The understanding of diagenetic effects on the seismic signal is of prime importance in the oil and gas industry because diagenetic bodies can act as barriers or pathways to hydrocarbons. These “diagenetic bodies” can form large reservoirs, which may be targets for exploration and their presence may also modify production strategies. Very subtle variations in seismic attributes provide a key to identifying these features. In this context, the calibration of seismic signatures with outcrops help constructing more realistic reservoir models based on amplitude inversion.

The present work documents forward-seismic models of decameter-to-kilometer scale depositional and diagenetic heterogeneities in a carbonate platform setting. The outcrop study was based on previously acquired high-resolution stratigraphical and sedimentological analyses of the “Nesque Valley” section (Monts-de-Vaucluse area). This section forms part of a huge Lower Cretaceous (Urgonian) carbonate platform, which developed mainly during the Late Barremian. The outcrop exposure is SW-NE oriented, perpendicular to the strike of the paleo-platform. This exceptional exposure enables an analysis of the evolution of proximal facies, represented by beach/inner plateform rudist-rich deposits, to outer shelf muddy cherts-rich sediments. Stratigraphic architecture consists of two facies patterns corresponding to two distinct depositional models separated by two major drowning events (mid Barremian and mid Bedoulian drownings). Diagenetic processes strongly overprinted the depositional architecture thus resulting in kilometer-scale microporous (up to 25% porosity) and tight bodies.

The model is 120 meters high and 4 kilometers long. Nine synthetic logs (referred to here as “pseudo-wells”) were constructed using 15 individual sections distributed along the entire 4 km section. Each “pseudo-well” was sampled at a metric scale. More than 80 samples were used for a detailed petrographic analysis and to construct a database of petrophysical measurements. Compressional wave velocities (Vp) and shear wave velocities (Vs) were conducted on dry-samples at five values of differential stresses (effective stress) that ranged from 0 to 40 MPa. Critical velocities, reached at 40MPa were used to deduce elastic moduli (bulk, shear, Young’s moduli and Poisson ratio). All these measurements were completed by density/porosity measurements.

Vp, Vs and density outcrop models were constructed using bi-linear (X-Z) interpolation conditioned to the pseudo-well data. Moreover, interpolations of these properties were performed independently in each “physical block” bound by well-established stratigraphic surfaces (unit-boundary/unconformity).

A Ray-tracing method was then used to construct seismograms after convolution with various frequency impulse wavelets (SEG Ricker-0-phase-30 Hz to SEG Ricker-0-phase-100 Hz). Since the model is only 120 meters high, the most efficient frequency chosen to distinguish the smallest bodies was the SEG Ricker-0-phase 100 Hz. This resolution is high compared to classical seismic resolution.

Models were constructed for two extreme cases: i) the first one was built considering only Vp, Vs and density measured from dry samples ii) the second one, based on the Gassmann’s relation, was performed using calculated values of velocities (Vp, Vs) and densities in the case of saturated samples with salt water (Sw=100%) to simulate fluid effect on seismic response.

High resolution seismic models clearly show that sequence boundaries or unconformities do not necessary correspond to changes in seismic signal. Acoustic impedance values in Monts de Vaucluse carbonates are mainly linked to porosity and pore type that are poorly controlled by the original depositional facies. Acoustic Impedance contrasts are related to diagenetic bodies boundaries and their seismic expression is function of porosity, pore type, tuning effect (and fluids content). Diagenetic overprint results in non-depositional reflector terminations and abrupt lateral polarity changes.

AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012