Sequence Stratigraphic Framework and Hydrocarbon Potential of Stylolite Distribution in Upper Cretaceous Kometan Formation, Western Zagros, Iraqi Kurdistan

Hussein HUSSEIN

Doctoral thesis

Abstract:

Surface analogues are suitable tools to link reservoir models to real facies, but they require a robust outcrop-to-subsurface correlation. This Dissertation thesis aims to characterize reservoir quality by statistically predicting electrofacies, fracture identification, statistically analyzing stylolite morphology and network distribution, and linking the reservoir to sequence stratigraphy from different Oil Fields in the Upper Cretaceous Kometan Formation, a prolific carbonate reservoir of northern Iraq. Conventional well logs (caliper, gamma ray, neutron, density, and sonic), along with geochemical data, were used in this study. Laboratory spectral gamma-ray measurements were also conducted. A multivariate regression approach was used to analyze electrofacies. Various interpretations, including porosity analysis and fracture identification, were employed. Field sampling quantified stylolite networks, and thin sections with scanning electron microscopy were used for pore estimation. The correlation was done between well logs and geochemical data from surface analogues. The results indicate that the Kometan Formation predominantly comprises limestone, marl, marly limestone, and argillaceous limestone. A statistically significant correlation (R2 = 0.83-0.85) between described and predicted lithology was established. The model with a higher coefficient of determination (0.85) was tested for further predictions in other wells. Six microfacies composed of mudstones to packstone with planktonic, and benthic foraminifers were identified in the outcrop, and interpreted as middle ramp, outer ramp, and basin deposits. The K+Al logs are correlated for 33 km, and the subsurface GR logs for over 100 km distance, but both reflect detrital admixture in biogenic carbonate. However, the outcrop- and subsurface log patterns show opposite vertical trends across the lower, middle, and upper parts of the formation. Such opposite log trends are interpreted as jointly reflecting landward and seaward shifts of the middle ramp carbonate factory within the mixed carbonate-siliciclastic system during transgressions and regressions, respectively. The inner ramp GR maxima and minima, and the outer ramp K+Al minima and maxima, are interpreted as maximum regression surfaces (mrs) and maximum flooding surfaces (mfs), respectively, in the T-R sequence-stratigraphic model. Neutron-density and sonic logs indicate that the best reservoir quality is achieved in fractured pure carbonates, which are controlled by these T-R cycles. Good secondary porosity has been Identified from the Secondary Porosity Index (SPI), which has reached a maximum value of 17%, and the average is 4% for Kz40. The maximum value is 30%, and the average is 8% for K216. In facies primarily composed of grain-supported packstone with moderate sorting, where grains are dispersed within a finer matrix, predominant stylolites typically exhibit weak connectivity and considerable vertical spacing. Conversely, mud-supported facies, marked by poor sorting and heterogeneity, display well-developed intersection stylolite networks. Stylolites in mud-supported facies demonstrate closer spacing, heightened amplitudes, and intensified junctions, with prevalent morphologies of the suture and sharp-peak varieties. Notably, this study unveils that stylolite significantly enhances porosity and permeability in the studied formation. Dissolution of minerals along stylolite seams forms intricate networks of channels and conduits. The results highlight the importance of the outcrop-to-subsurface correlation of well log data and elemental geochemistry in stratigraphic correlation and reservoir quality.

Abstract:

Surface analogues are suitable tools to link reservoir models to real facies, but they require a robust outcrop-to-subsurface correlation. This Dissertation thesis aims to characterize reservoir quality by statistically predicting electrofacies, fracture identification, statistically analyzing stylolite morphology and network distribution, and linking the reservoir to sequence stratigraphy from different Oil Fields in the Upper Cretaceous Kometan Formation, a prolific carbonate reservoir of northern Iraq. Conventional well logs (caliper, gamma ray, neutron, density, and sonic), along with geochemical data, were used in this study. Laboratory spectral gamma-ray measurements were also conducted. A multivariate regression approach was used to analyze electrofacies. Various interpretations, including porosity analysis and fracture identification, were employed. Field sampling quantified stylolite networks, and thin sections with scanning electron microscopy were used for pore estimation. The correlation was done between well logs and geochemical data from surface analogues. The results indicate that the Kometan Formation predominantly comprises limestone, marl, marly limestone, and argillaceous limestone. A statistically significant correlation (R2 = 0.83-0.85) between described and predicted lithology was established. The model with a higher coefficient of determination (0.85) was tested for further predictions in other wells. Six microfacies composed of mudstones to packstone with planktonic, and benthic foraminifers were identified in the outcrop, and interpreted as middle ramp, outer ramp, and basin deposits. The K+Al logs are correlated for 33 km, and the subsurface GR logs for over 100 km distance, but both reflect detrital admixture in biogenic carbonate. However, the outcrop- and subsurface log patterns show opposite vertical trends across the lower, middle, and upper parts of the formation. Such opposite log trends are interpreted as jointly reflecting landward and seaward shifts of the middle ramp carbonate factory within the mixed carbonate-siliciclastic system during transgressions and regressions, respectively. The inner ramp GR maxima and minima, and the outer ramp K+Al minima and maxima, are interpreted as maximum regression surfaces (mrs) and maximum flooding surfaces (mfs), respectively, in the T-R sequence-stratigraphic model. Neutron-density and sonic logs indicate that the best reservoir quality is achieved in fractured pure carbonates, which are controlled by these T-R cycles. Good secondary porosity has been Identified from the Secondary Porosity Index (SPI), which has reached a maximum value of 17%, and the average is 4% for Kz40. The maximum value is 30%, and the average is 8% for K216. In facies primarily composed of grain-supported packstone with moderate sorting, where grains are dispersed within a finer matrix, predominant stylolites typically exhibit weak connectivity and considerable vertical spacing. Conversely, mud-supported facies, marked by poor sorting and heterogeneity, display well-developed intersection stylolite networks. Stylolites in mud-supported facies demonstrate closer spacing, heightened amplitudes, and intensified junctions, with prevalent morphologies of the suture and sharp-peak varieties. Notably, this study unveils that stylolite significantly enhances porosity and permeability in the studied formation. Dissolution of minerals along stylolite seams forms intricate networks of channels and conduits. The results highlight the importance of the outcrop-to-subsurface correlation of well log data and elemental geochemistry in stratigraphic correlation and reservoir quality.

Keywords

sequence stratigraphy, stylolite, fracture, well logs, element geochemistry, Kometan Formation

Language used: English

Date on which the thesis was submitted / produced: 16. 5. 2024

Thesis defence

• Supervisor: prof. Mgr. Ondřej Bábek, Dr.

Palacký University Olomouc

Faculty of Science

Doctoral programme / field:
Geological Sciences / Geological Sciences