Regional mapping indicates that the regional stratigraphically discordant dolomite bodies cross-cut formation boundaries. The dolomite volume is most pervasive adjacent to the intrashelf salt basin and decreases with distance to the north (Fig. 3.1).
Three-dimensional (3D) geostatistical modeling provides a comprehensive description of subsurface reservoirs. It is an effective way of characterizing subsurface reservoir architecture, facies geometry and reservoir properties mainly based on field data such as well logs, cores and seismic data (Rodriguez 1988; Zu et al., 2012). Since late 1990s, the technology of reservoir modeling integrated with seismic data has been developed and widely used in reservoir characterization. Araktingi et al. (1992) discussed a workflow of using seismic data in reservoir characterization and fluid-flow predictions integrated with well logs. Yang et al. (1995) discussed about incorporating seismic acoustic impedance with well log derived porosity to generate the porosity model in a gas field. Strebelle et al. (2002) employed multiple-point geostatistical technique in describing a deep water turbidite reservoir under the condition of using seismic data. Michelena et al. (2009) established a workflow to estimate facies probabilities from log and seismic data and use the information to constrain reservoir facies modeling.
A stochastic geological model was built to characterize the massive dolomite distribution and predict the dolomitization effect on reservoir quality using Sequential Gaussian simulation algorithm. The model covers an area of about 6.6 x 103 km2 with 500 m by 500 m grid spacing in X and Y direction. The modeling interval is consist of 132 layers of 10 zones, in a total number of 3.9 x 107 cells.
Structural model shows the topographic elevations generally increase from east to west (Fig. 3.2). The massive dolomite mainly occurs in the northeastern part of the study area, with its maximum thickness in the “dolomite center” near the southern margin of the intrashelf salt basin to the north. Towards the south and west, the dolomitized intervals progressively decrease in thickness to the “dolomite front,” forming a wedge-shaped geobody (Fig. 3.3).
Properties modeling results also prove the proposed two phase’s dolomitization model. Figure 3.4 show the dolomite content, porosity and permeability change along the cross section from “dolomite center” to “dolomite front.” Areas with high dolomite content tend to have relatively low porosity and permeability, also supported by thin sections.
To further exploring of tight dolomite sealing potential, a detailed dolomite model was performed by integrating seismic properties for a specific interval (interval A) within the study area (Fig. 3.5). The grid spacing in X and Y direction is 100 m by 100 m. The total number of cells is about 1.1 E x 107.
As shown in Fig. 3.6, dolomitization potential increases from grainstone to mudstone, very well matches the proposed model. Non reservoir or seal can be defined by porosity cut-off (mudstone) among which tight dolomite seal has a dolomite volume fraction approximately larger than 0.8.
Figure 3.7 and 3.8 show relatively good correlations among porosity, acoustic impedance (AI) and dolomite content (). Therefore, we adopted AI as the additional data source to the dolomite model and porosity model. A number of algorithms were tested and co-kriging method was selected to generate the models with and without AI constrain (Fig. 3.9).
Overall, dolomite is well developed in interval A especially in the northwest area. Limestone and dolomitized limestone with relatively lower dolomite content have good reservoir quality in the eastern part of the study area. The tight dolomite developed in pore-fill phase as well as some muddy dolomitized limestone have good potential of acting as lateral (Fig. 3.10). This configuration presents a good reservoir-seal combination, which constitutes a diagenetic play concept (Fig. 3.11).
CONCLUSIONS
Stratigraphically discordant massive dolomite bodies have long been observed and documented in the study area because of its significant impact on reservoir quality. Dolomite can behave as either a conduit or a barrier to flow depending on the original depositional texture, chemistry of dolomitizing fluid, diagenesis stages, types of dolomitization, and presence of anhydrite.
The dolomite volume fraction-porosity relationships suggest that (1) generally, dolomitization can be divided into replacement and pore-filling (over-dolomitization) phases; (2) porosity preservation characterizes the former phase, and reduction of porosity for the latter phase; (3) grain-dominated limestone facies usually have less dolomitization potential (e.g., the dolomite fraction is usually < 0.8), whereas muddy facies have a higher dolomitization potential. Away from the dolomitizing fluid source, low flux and normal concentrations cause porosity preservation. Near the fluid source, high flux and high concentrations cause over-dolomitization. Tight dolomite can be an effective seal featuring high dolomite content, small pore throats, low porosity and low permeability.
A detailed dolomite model of a thin layer of interval A within seismic coverage indicates that there is a good potential for diagenetic trap, which formed by porous limestone reservoir and up-dip tight dolomite and muddy dolomitized limestone seal in this heavily dolomitized study area.