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Understanding the relationship between depositional facies, diagenetic evolution and reservoir character in The Cretaceous Shuaiba and Natih Formations of North Oman |
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| รหัสดีโอไอ | |
| Creator | Jiraporn Pidnoi |
| Title | Understanding the relationship between depositional facies, diagenetic evolution and reservoir character in The Cretaceous Shuaiba and Natih Formations of North Oman |
| Contributor | - |
| Publisher | Department of Geology, Chulalongkorn University |
| Publication Year | 2557 |
| Journal Title | Bulletin of Earth Sciences of Thailand (BEST) |
| Journal Vol. | 7 |
| Journal No. | 2-Jan |
| Page no. | 76-83 |
| Keyword | Shuaiba, Natih, Cretaceous carbonate, Oman |
| URL Website | https://www.bestjournal.org/ |
| Website title | Bulletin of Earth Sciences of Thailand |
| ISSN | 1906-280X |
| Abstract | The Shuaiba and Natih Carbonate Formations are the most important petroleum reservoirs in North Oman. The reservoir character is significantly heterogeneous and the entrained hydrocarbons accumulated in a complex suite of pore types, so as a result it is a reservoir association that is difficult to produce. This is a major challenge to the economics of exploration and enhanced reservoir recovery programs, focused on these complex reservoirs. To help resolve some of the complexity a study of core from well that intersects both the Natih and the Shuaiba was done. It uses an integrated combination of stable-isotope data, petrographic study and core observations to construct the diagenetic and poro-perm history and so evaluate the related influences of depositional facies and diagenetic evolution on porosity and permeability distribution in well A. Reservoir properties of both formations are controlled by different pore response/ mechanical-strength responses, which can be recognized from the different poro-perm relationships in the two formations. The Natih is shallower than the Shuaiba, but has a less consistent poro-perm trend, which is likely caused by a wider range of grain and crystal sizes. Natih is comprised of lagoonal bioclastic shales and open marine skeletal wackestones/packstones, which are finer grained, with higher mud contents and lesser corroded and friable fragments than the Shuaiba. Cm to dm-scale grain-rich layers in the Natih carbonates are generally encased by mudstones. Therefore, the contrast in mechanical stress response between shale and grain-rich intervals is obvious in Natih core. Groups of microfractures are obvious in the formation, especially in association with mud-rich zones. In contrast the Shuaiba is mainly comprised of upwards-coarsening rudist rudstone/floatstone units that were deposited in current-reworked rudist banks or shoals. The poro-perm trend of Shuaiba core-plug samples is similar to that seen in many siliciclastic reservoirs; high porosity results in high permeability, although the trajectory of this poroperm trend is somewhat steeper than a typical sandstone do to the overprint of a late-stage corrosive leaching event. Even so, reservoir quality of the Shuaiba is likely defined by a somewhat more predictable relationship between depositional facies and the diagenetic history, compared to the Natih. Some stylolites and isolated microfractures were observed in Shuaiba, but microfractures are more obvious in the Natih. Integration of core observations, petrography and texture-aware isotope study reveals a diagenetic history that in the earlier to moderate stages of burial is thermally similar for both the Natih and the Shuaiba formations. Throughout the carbon values in the Natih are more influenced by the higher levels of organics, inherent to its lagoonal depositional setting. Only the Shuaiba shows evidence of a late stage leaching event leading to creation of high porosity vuggy intervals These late stage vugs have spar-cement linings with ?18Opdb values that are consistently more negative (warmer) than -6. Equivalent vugs and cements are not present in the Natih Formation interval in Well A. When the isotope plotfields of the Shuaiba Formation in well A are compared with isotope values in time equivalent Shuaiba reservoirs in the highly productive oilfields of the nearby UAE, it is apparent that the Shuaiba formation in this part of Oman did not experience a flush of syndepositional and early diagenetic meteoric waters. This early flush of undersaturated waters created high levels of karstic porosity in the UAE rudist build-ups, which now hosts the hydrocarbons in the various Shuaba-reservoired giant oilfields. The lack of this isotopic signature in the Shuaiba Formation in well A implies a new exploration paradigm, perhaps tied to structural evolution and the timing of fault related fluid conduits, should be developed for this part of northern Oman |
