English abstract
The Natih Formation was deposited from the Late Albian until the Early Turonian in Oman, Middle East. It is the lateral equivalent to the Mishrif Formation in the UAE and to the Sarvak Formation in Iran, and belongs to one of the most productive oil-bearing carbonate interval in the world. Within this stratigraphic unit, several giant fields are currently in production such as the Fahud field in Oman. The detailed stratigraphic architecture of the Natih Formation and the level of heterogeneity within each reservoir unit are very important to constrain and quantify, since they may have a significant impact on fluid flow and oil-recovery, The Natih Formation corresponds to a very broad, extensive inner carbonate platform that developed over more than 1000 km between the Arabian Shield in the south-west and the Tethys ocean-facing platform margin in the north.. The Natih Formation corresponds to three fully-developed third-order sequences showing the same evolution of depositional system: from a mixed clay-carbonate tabular system at the base to a carbonate-dominated ramp system dominated by rudists and benthic foraminifera above (van Buchem et al., 2002). Intrashelf basins (30 to 40 m deep) developed by differential aggradation within sequence I and sequence III during the major transgressive events of the Late Albian and Late Cenomanian. These three sequences are bounded by one or several exposure surfaces, along which small incised valleys (incisions) can develop.
Because it is overall an aggrading carbonate platform system, the third-order sequences appear very tabular and isopach, which often leads to "layer-cake" correlations, even at the scale of the reservoirs. However, within these third-order sequences, a high level of sedimentary and stratigraphic heterogeneity can develop, and they are linked with the development and fill of intrashelf basins (inclined time lines), phases of subaerial exposure, and phases of flooding of the platform. The present thesis focuses on the study of these heterogeneous stratigraphic intervals.
The methodology used is an integration of analysis at different scales with different techniques. These stratigraphic intervals are studied in detail on several outcrops of the Oman Mountains (Adam Foothills and Al Jabal Al Akhdar). The regional stratigraphic framework is reviewed and extended by regional 3D seismic interpretation. A particular focus is made on specific sediment bodies (channelized structures, sedimentary wedges) on both outcrops and seismic data and the technique of Synthetic Seismic Modelling is used to refine the seismic interpretation of these sediment bodies. Constant iterations between these different observation, outcrop analysis and seismic interpretation, were made in order to build the most accurate sedimentological and stratigraphic model for the Natih Formation.
The results are summarized below:
Heterogeneities linked with the development and fill of intrashelf basins Within the first third order sequence of the Natih Formation, the transgressive systems tract is characterized by tabular to very low angle ramp geometries (progressively increasing, up to 0.3%). These low-angle units are mostly made of muddy carbonate facies and grade downward to organic-rich basinal facies. The highstand systems tract is characterized by progressively steeper sigmoid clinoforms (0.49 to 1). The topsets of these clinoforms are made of aggrading lagoonal facies with nudists and corals in situ, the ramp margin (at the offlap break) is made of high-emergy bioclastic grainstone facies, and the outer ramp is made of muddy carbonate facies with chert nodules in the upper part. The direction of progradation is towards the west in the cast (Adam Foothills area) and towards the east in the west (Fahud field area) Heterogeneities linked with phases of subaerial exposure In the Natih Formation, phases of subaerial exposure are recorded by the development of incisions on the exposed carbonate platform and by forced regressive wedges on the margins of the platform. Incisions are sinuous elongated sediment bodies which are one kilometre wide, twelve-to twenty-metres thick and forty- to more than eighty-kilometres long. They are filled during a transgressive phase with three successive and different fill units. The first part of the fill is to a high-energy lag deposit (good reservoir facies). The second and main part of the fill corresponds to low-energy confined deposits either made of mudstone or green clays (poor to non-reservoir facics) and the last part of the fill corresponds to higher energy deposits, usually organized as large low-angle inclined stratifications complex with stacked reservoir and nonreservoir facies). Forced regressive wedges are "high angie" (20 to 5°) oblique clinoforms mainly made of an alternation of rudstone and floatstone with large rudist fragments. They have a limited 3D extent (15 x 5 km).
Heterogeneities linked with phases of flooding of the platform highly sinuous elongate
In the Natih Formation, phases of flooding of the platform are characterized by the development of tidal channels and associated bioclastic sandwaves and shoals. Tidal channels form sinuous to highly sinuous elongate sediment bodies which are one kilometre wide, three to ten metres thick and twenty to thirty kilometres long. Channel fill and sandwaves are mainly made of course high-energy facies (good reservoir facies) organized as a complex set of carbonate bioclastic sediment bodies, generally corresponding to laterally accreting bedsets with low to high angle inclined stratifications (up to 20%). metres long, Channel fill and suncomplex set of The results of this study have been used to refine the high resolution stratigraphic model of Natih sequence and sequence II in the Fahud field. The integration of 3D seismic interpretation, core study, correlation with outcrops, regional stratigraphic framework and forward modelling has allowed to correlate the Fahud wells with time lines which may better define flow units than lithostratigraphic boundaries. These time lines appear to be more inclined than expected and even downlapping between the wells, since force regressive and lowstand wedges have been interpreted within sequence I. Heterogeneous intervals such as the upper part of Sequence 2, corresponding to the maximum flooding interval, have also been highlighted. A significantly more detailed geological model could then be built for these two sequences, upon which to base the static reservoir models required to assess the re-engineering potential of this mature giant field.
