Late kimmeridgian carbonate ramp (Jabaloyas, ne Spain): sedimentological model review and implications for hydrocarbon reservoir static modelling
- Autores: Galo San Miguel
- Directores de la Tesis: Marcos Aurell Cardona (dir. tes.), Beatriz Bádenas Lago (dir. tes.)
- Lectura: En la Universidad de Zaragoza ( España ) en 2017
- Idioma: español
- Tribunal Calificador de la Tesis: Jeroen A.M. Kenter (presid.), Ana Rosa Soria de Miguel (secret.), Idoia Rosales (voc.)
- Programa de doctorado: Programa Oficial de Doctorado en Geología
- Materias:
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- Resumen
The results presented in this PhD thesis are based on extensive and transverse stratrigraphic and sedimentological analysis, 3D modelling and quantification, geophysiscal and diagenetic studies of the upper Kimmeridgian outcrops exposed around the village of Jabaloyas (NE Spain). The studied unit corresponds to a high-frequency sequence bounded by regional discontinuities and allows the characterization of the shallow domains on a low angle carbonate ramp. Along this shallow ramp, a wide spectrum of grain- to mud-supported facies (skeletal, peloidal, oolitic, intraclastic) coevally developed into microbial to coral-microbial buildups with pinnacle morphologies up to 19 m-high.
The depositional facies and the stacking patterns in the Jabaloyas outcrops show certain similarities with those described in the subsurface Arab D reservoirs. As a consequence, the outcrop characterization of the vertical and lateral sedimentary facies distribution observed in the upper Kimmeridgian outcrop of Jabaloyas helps to better understand and predict the sedimentary heterogeneities, as well as certain aspects of the diagenetic overprint, documented in the Arab D reservoirs. The studied area covers a total surface of 12 km2 and provids critical reservoir-scale data.
The facies heterogeneity across the shallow portion of the carbonate ramp is documented through extensive field analysis including the measurement and sampling of 17 stratigraphic profiles. The characterization and interpretation of inter-reef and post-reef facies resulted in the reconstruction of the sedimentary domains, from bioturbated mud-dominated skeletal facies (distal middle ramp), to grainy (skeletal, peloidal, oolitic) shoal and backshoal facies (inner ramp). All the documented facies have been interpreted as deposited in shallow subtidal environments, in an almost pure carbonate system, practically free of any clastic input. To improve the understanding of geological heterogeneity in this type of low angle carbonate ramp systems, two 3D models were generated using Petrel® software. A Full Field Model of the overall geometry of the facies belts allowed the quantification of vertical and lateral extension of the inter-reef and post-reef facies (20-m grid). A Sector Model, using an object-based modelling approach, including individual reef bodies, was used to optimize the simulation of reservoir heterogeneities (1-m grid).
Object-based modelling was used to upscale the pinnacle buildups within the model. More than 270 buildups were georeferenced and described in terms of thickness and width at the well-exposed outcrops windows. Besides, a geomorphological platform allowed direct measurement in facies of magnetic susceptibility and detailed grids of magnetometry, electromagnetic multi-frequency and ground-penetrating radar (50-500 MHz antennas). Magnetometry indicates negative anomalies in residual magnetic field and vertical magnetic gradient related to reef pinnacles and faults. Electromagnetic (EM) data reveal that positive anomalies of apparent conductivity correlated with non-reefal facies. However, the areal distribution of magnetometry and EM data does not permit the unequivocal identification of pinnacles and faults. In contrast, ground penetrating radar profiles and maps of relative reflectivity in two way travel time slices were useful for the identification of faults (hyperbolic anomalies) and reefal and non-reefal facies (radar facies A and B, respectively). The integration of geophysical data, mainly ground penetrating radar, has permitted the 3D reconstruction of reef pinnacles and its associated tectonic framework.
The factors controlling the nucleation and growth of different types of bioconstructions in the shallow carbonate ramp were also examined. The facies architecture from a specific outcrop windows exposed in the Barranco del Diablo allowed the definition of the spatial and vertical transitions of a wide range of bioconstructions, including pure microbial buildups, coral-bearing thrombolite buildups, coral-microbial buildups, oyster patches and stromatoporoid carpets. Some buildups are stacked and forming pinnacles, 10–19 m thick, within a broad spectrum of coeval inter-reef carbonate facies. Coral-bearing thrombolites are coincident with oolitic grainstones indicating shallow marine conditions developed during the initial platform flooding. During the progressive sea level rise, coral-microbial buildups occurred in proximal to distal mid-ramp domains, where the proportion of microbial crust ratio tends to increase with depth. Inter-reef oolitic facies sharply grades down-dip to hummocky cross–stratified intraclastic, peloidal and skeletal deposits, mostly sourced from the coral-microbial buildups. During the first regressive pulses, deposits were dominated by pure microbialites (encrusted by Tubiphytes-Crescentiella and serpulids) in the proximal areas, related to local fresh water input involving seawater stratification and oxygen depletion. The upper part of the unit indicates an initial recovery of the metazoan builders, with abundant branching corals. Well-oxygenated with high nutrient concentrations at the last stages, with Marinella-lugeoni red algae, oyster patches and stromatoporoid boulders developed close to the shoreline environment. The reported data lead the discussion towards the optimal environmental conditions for each “reef window” in Jabaloyas, such as sediment and nutrient supply, water depth, wave energy and light availability.
Interpretation of the highly variable burial versus porosity curve in carbonate subsurface reservoirs requires a full understanding of their complex diagenetic stories. The porosity evolution through burial must be established in order to limit the hydrocarbon charging timing into the reservoir. Reservoir modelling involves not only the characterization of the main drains (highly connected permeable layers in the bests reservoir areas), but also the barriers (continuous low permeability layers; baffles) that might compartmentalize the reservoir drop either along the vertical and/or the horizontal axis. Fossil and modern analogues help us to decipher, for a determined stratigraphic interval and geographic location, what it might have been developed in the sediment-diagenesis interphase (eogenesis) or later, during mesogenesis (deep burial phase). A combined number of methods (i.e. petrographic microscopy, cathodoluminiscence, Qemscan® analysis) were applied to describe the main fabric elements within a paragenetic sequence along with the observed mineralogy, carbonate cements, porosity network, compaction elements and fissure-types of the Jabaloyas deposits. The ultimately goal was to provide guidelines for establishing the diagenetic processes creating (poro-enhancing) and/or plugging (poro-necrosing) porosity. The outcrops studied in Jabaloyas suggest that the cementation and dissolution issues at early stages of the diagenetic history, before compaction take place. Precisely, the studied high-frequency Sequence C would represent an entire catch-down sequence with an incomplete shallowing-up succession. But higher order cycles might get the sediment closer to the sea level where meteoric and marine waters might mix, triggering early dissolution processes of aragonitic skeletal components such as corals, stromatoporoids among others.
