Conclusions and Comparisons

Summary and Findings
The primary objective of this study was to use the two geographically distant carbonate basins to improve our understanding of how carbonate sedimentary systems work. Two carbonate basins, the Upper Jurassic Arabian Basin of Eastern Saudi Arabia and the Upper Permian Basin of West Texas and Southeast New Mexico were compared to try to establish the major controls responsible for the carbonates sedimentation characteristics. A brief overview of the structural development of each basin was also examined to see if it was possible to discern the primary events that lead to the development of these basins and to the eventual entrapment of the vast hydrocarbon reserves that have been discovered in both these basins. Additionally, a brief examination of the history of oil exploration and production for the basins was examined. A more detailed study of the Hanifa formation of the Arabian Basin was conducted using 14 wells provided by Saudi Aramco.

The prolific oil and gas fields of Saudi Arabia (northeastern margin of the Arabian Plate) are linked to the margin's long and stable tectonic history (Beydoun, 1991). The Arabian Plate was a virtually flat platform that has been coupled with an almost uninterrupted accumulation of sheets of thick and uniform sediments. Various tectonic events both during deposition in the Late Jurassic and during the collision of India with Iran in the late Cretaceous and early Tertiary lead to the formation of gentle structural traps that captured the hydrocarbons generated in the region.

In summary, Arabian platform is comprised of Paleozoic intraCratonic basins overlying crystalline basement. The Mesozoic basins formed as the results of Late Permian and Early Triassic opening of the adjacent Neo-Tethys Ocean and a general major rise in eustatic sea level. This resulted in the development of the Arabian basin over the high-energy passive margin (bounded by northern Rimthan Arch, eastern continental margin, and southern Qatar Arch). Very thick high-energy reservoir-prone carbonates accumulated on the northern shallow shelf, while source-prone deep-water lime mudstones were deposited to the south and in the southwest regions of the basin. Later tectonic events lead to the deformation and compartmentalization and eventual formation of traps containing the prolific hydrocarbon reservoirs of the region.

The hydrocarbon reserves of the Permian Basin of West Texas and New Mexico were found in the shelf carbonates and the basin sandstones. Shelf and basin topography were the direct product of the basin differentiation caused by Hercynian Orogeny in which the North American Craton collided with the South America from Early Pennsylvanian through Early Permian. The result was the deep basins containing lowstand sandstone deposits that were surrounded by shallow shelves that were the sites for the deposition of transgressive and highstand carbonate deposits. The San Andres formation carbonates represent the primary hydrocarbon reservoirs of the Permian Basin.

The analysis of the wells for the Hanifa formation confirmed and added to the conclusions of previous work. This was that the bulk of the Hanifa reservoir is composed of skeletal conglomerates lithofacies deposited on the relatively high-energy shelf (northern region of the formation) during the highstand systems tract of the upper Hanifa sequence. It was also determined that the contemporaneous organic-rich lime mudstones were deposited within the southern region of the basin when a late transgressive systems tract and the highstand systems tract developed under quiet and anoxic water conditions. The lithofacies shifted from the shelf to basin gradually and grades through the identified eleven lithofacies in response to the changes of the relative sea level and the basin depositional profile (gentle ramp with an intrashelf basin to the south) on which they accumulated. The lithofacies distribution in the basin enables the prediction that the best reservoir facies occur mainly to the north and are interpolated with intermediate reservoir lithofacies.

The Hanifa formation consists of two transgressive-regressive (shallowing-upward) third-order sequences within a second-order transgressive event. These are represented by the lower Hadriya and the overlying Hanifa sequences. This study was focused on the Hanifa sequence. Each of these sequences is bounded by a drowning transgressive surface (TS) with the bulk of each reservoir rocks consisting of lower keep-up aggrading and prograding grainstones of a highstand systems tract that are overlain by a sub-aqueous boundaformformformry. The transgressive systems tract is identified by the back stepping grainstones onlapping the shelf-margin wedge systems tract.

Source rocks of the Hanifa formation were deposited within the transgressive and highstand systems tract and coincided with the TST limbs of the first, second, and third order sea level cycles that formed during the maximum rate of sea level rise of sea level in the Phanerozoic. This major rise in sea level provided the necessary conditions for increased phytoplankton production and preservation of freshly produced organic matter in the intrashelf basin. The general lithofacies trend is of decreasing porosity and permeability southwestward from skeletal grainstones to the north to dense laminated lime mudstone in the south. The lowstand systems tract of the second-order sequence caused the precipitation of regional evaporitic seal throughout the basin. Third-order seals were either dense lime mudstones in the lower sequences changing to evaporitic seals over the Jubaila and the Arab members.

The stratigraphic section was subdivided into shoaling upward cycles. Those cycles are most recognizable in the shallowest areas of the field. These shallow cycles are also useful for detecting relative-sea level fluctuation on a local scale and so can then be used for regional correlation. This is especially crucial since the gamma log character was most ambiguous in the shallow regions and relying on a cycle character that was the only means (beside looking at cores) for sequence stratigraphic correlation and interpretation. The LST section is distinguished by means of the prograding thick cycles that thin upward and basinward and onlap the sequence boundary (SB-2). These cycles are thickest in basinal region. The TST section is distinguished by means of the aggradation of the deepening-upward cycles, that are generally uniform in thickness but thin upward. The HST section is distinguished by the initially thin cycles that aggrade then prograde and shallow and thicken upward. These cycles converge on shelf toward the end of the HST. The HST section represents the bulk of the sequence.

Fischer plots were drawn for two wells, one selected from the shallowest parts of the section (well # 233) and the second from the central part of the section (well # 006). Selecting wells from the deeper part of the section was not useful since they contained only condensed section facies. These diagrams for the shallower section enabled the reconstruction of the sea level in the peritidal shelfal regions and in general mimicked the Haq eustatic sea level curve. The diagrams also revealed three fourth-order sequences that were not identified earlier. Each of these sequences is bounded by a type-2 sequence boundary and is recognized on the diagram at the maximum sea level fall inflection point. An important observation is that the maximum flooding surface (MFS) that was identified was based on the recreated sea level curve (at the point of maximum cumulative cycle thickness) has a different position from the MFS identified on the section.

 The Permian Basin is characterized by a progressive shallowing upward trend. The depositional profile of the basin reflects this trend of shallowing with a change from an initially open distally steepened ramp into a restricted sigmoid-progradational ramp, exposed platform, transitional ramp to rimmed shelf, and finally into mixed clastic-carbonate reef-rimmed shelf. This increased steepness in the shelf-margin is a direct result of the basin topography being coupled with the shallowing trend, which caused the shelf-margin deposits to keep-up with rising sea-level and prograde basin-ward.

Lowstand systems tracts resulted from a lowering in the relative sea level and occasional exposure of the peritidal region or even the shelf margin sections of the basin. The result was deposition of basinal linear channel sandstones that were supplied through shelf bypass and the accumulation of aeolian siltstones. The slope is characterized by carbonate slumps from the exposed shelf margin facies. In some places the shelfal LST lithofacies were identified as onlapping onto the shelf margin carbonates and by progradation of sandstone tongues that moved southward toward the basin. The tidal flats of the LST contain mostly aeolian sandstones and siltstone that overlie the underlying supratidal lithofacies of the TST and HST. The basin fill is generally composed of thin carbonate beds that alternate with thickening-upward sandstone and siltstone beds and onlap the shelf with thick beds of sandstone.

Transgressive systems tracts resulted from abrupt deepening of the basin and gradual reestablishment of carbonate production. This means that TST facies are primarily recognized by the deepening-upward of the lithofacies succession. This change occurred throughout the basin but is most pronounced at the shelf margin where the carbonate factory was turned on and kept-up with rising sea level. Gradual retrogradation depositional patterns became established as carbonates overlying the LST sandstones in the basin and slope occurred as thinning-upward layers of burrowed wackestone and even oxygen-poor lime mud in the deepest part of the basin. The tidal flats TST are characterized by supratidal facies of arid and hot settings including fenestral pisolitic dolomudstones and dolopackstones. The carbonates of the shelf margin developed into a steeper feature while the sandstone beds in basin became thinner.

The highstand systems tract was established as a result of a slowing down in the rise of relative sea level and is marked by keep-up carbonate production on the shelf margin and domination of carbonate deposition throughout the basin. The HST sequences are generally composed of shoaling upward cycles that gradually changed from aggradational during early HST to progressively progradation as a result of accommodation space being filled. The general lithofacies distribution of the HST is of thick beds of carbonates on shelf and shelf margin and very thin beds of sandstone in the basin and slumps on slope. The parasequences progressively become more restricted towards the end of the sequence with evaporite beds precipating in basin in response to this restriction while red beds accumulated on the shelf.

Comparison
Table 10.1 provides an overview of the major differences between the Arabian Basin and the Permian Basin, summarizing the findings of this study.

Property

Arabian Basin

Permian Basin

Formation

Hanifa, Jubaila, and Arab (A-D) represented by shallowing-upward marine depositional cycles. Each cycle started with shallow-water, normal marine carbonate and closed with the accumulation of nearly pure anhydrite. The cycles are progressively regressive at they approach the end of the 2nd order sequence. The Hanifa reservoir facies occur on the shallow shelf while source-prone rocks are in the deeper parts of the intrashelf basin. Upper reservoirs are on shallow shelf.

San Andres (lower, middle, and upper) carbonates of the Northwestern and Eastern Shelves and the Central Basin Platform. Clastics form significant hydrocarbon reservoir in the Delaware Basin. Locally, the carbonates serve as seals and source rocks. Significant reason for loss of reservoir quality is plugging by evaporites. Oil and gas fields occur on the shelf and commonly immediately downdip of the zero porosity line (a result of evaporite plugging).

Location

Eastern Arabia

West Texas and Southeastern New Mexico

Age

Upper Middle to Late Jurassic (late Oxfordian and Kimmeridgian) (150 m.a.)

Late Permian (Lower Guadalupian) (260 m.a.)

Platform

Gentle carbonate Ramp

Distally steep carbonate ramp transformed into rimmed shelf.

Climate

Arid (rain shadow from continental mass to west)

Arid (rain shadow from surrounding continental areas)

Margin (tectonic)

Shelf Shoal on pull apart Tethyan margin over Hercynian block faulting

Shelf of interior back arc basins Hercynian block faulting

eustatic Signal

Order changes in base level above third order are product of small (approx. 1-2 m) amplitude Malenkovitch climatic perturbations

Order changes in base level above third order are product of large (20-30 m; Ross & Ross, 1987) amplitude glacial eustatic perturbations

Response to sea-level rise - Geometry

(Highstand) Keep-up sheets carbonate (sheet-like geometry formed during sea-level still-stands and rises)

(Highstand) Keep-up grainstone sheets aggrade landward and basinward from the shelf margin.

Rate of carbonate production

Hanifa is 72 um/yr based on 152 m in 2 m.y.

Lower/Middle San Andres: 280 um/yr
Upper San Andres: 180 um/yr

Average Thickness

Hith: 167 m; Hanifa: 152 m; Arab: max thickness is 430 m.

Lower/Middle San Andres: 280 m
Upper San Andres: 180 m.

Parasequence character during 3rd order LST

Prograding thick cycles thinning-upward and basinward and onlapping the sequence boundary (SB-2). Thickest in basinal region.

Shelfal sandstone cycles onlapping the shelf margin and prograding downslope. The tidal flats clastics onlap supratidal lithofacies of the vertically adjoining TST & HST. The basin fill is generally composed of thin carbonate beds alternating with thickening-upward sandstone and siltstone beds and onlapping the shelf with thick beds of sandstone.

Parasequence character during 3rd order TST

Thin aggrading of deepening-upward cycles, generally uniform in thickness but thins upward.

retrogradational deepening-upward cycles and re-establishment of carbonates on underlying the LST sandstones in the basin and slope and unconformity on the shelf. Thinning upward sand layers in the basin.

Parasequence character during 3rd order HST

Initially thin cycles aggrading then prograding, shallowing and thickening upward. Converges on shelf toward the end of HST. Represents bulk of section.

Shoaling upward cycles that gradually changed from aggradational during early HST to progressively progradational. Thick beds of carbonates on shelf and shelf margin and very thin beds of sandstone on basin and slumps on slope. cycles progressively become more restricted toward end of sequence.

Reservoir

Porosity: 7-17%; permeability <100md

Porosity: 7-15%; permeability <10md

Source

Hanifa: anoxic, organic-rich carbonate of Callovian to Oxfordian interval. Total organic content averages 1-6 wt. %;

Calcareous shale and shaly limestone of Wolfcampian and Leonardian age both basinal and lagoonal and to a lesser extent the Late Devonian Woodford shales.

Seal

The regional Hith formation and evaporitic seals on top of each member.

Regional Salado and Castile formations and up-dip sabkha San Andres evaporites; tight carbonates and evaporitic plugging.

Reservoir systems tract

Highstand shelf carbonates of 3rd order sequence during HST of 2nd order sequence.

Highstand shelfal carbonates during increasingly regressive HST of 2nd order Guadalupian sequence. Lowstand basinal sandstones.

Source systems tracts

TST basinal lime mudstones of 3rd sequences during TST of 2nd order sequence.

Transgressive basinal and lagoon lime mudstone.

Seal systems tract

Transgressive dense lime mud during 2nd order TST and lowstand evaporites during the later regressive sequences of the 2nd order sequence.

lowstand wedges and transgressive evaporites deposits?

Reservoir facies

High-energy and shallow water e porousooidal-pelletoidal and skeletal conglomerates deposited just below sea level.

Shelfal, high-energy grain carbonates at the top of the shoaling upward cycles, which were altered in some places by diagenesis and dolomitization. Clastics of the Delaware Basin.

Source facies

Organic-rich laminated mudstones deposited low-energy, below storm-based water depth.

Organic-rich calcareous shale and shaly limestone.

Seal facies

Tight lime mudstone in the lower transgressive sequences changing to evaporites in the upper regressive sequences.

Table 10.1 - Arabian Basin & Permian Basin comparison.


Future Work

A more in-depth analysis of the Eastern Arabian Basin should be possible if more well data were available. In this case study, objectives could include the construction of porosity models from which the prediction of lithofacies, and the construction of 3-D lithofacies models might result. Additionally, a more precise sea level curve based on Fischer diagrams might be constructed if more wells were available in the shallow shelf region of the basin (north). A tie to the outcrops of the Hanifa in the Tuwaiq Mountains would detail the sedimentary history of this formation.

A more thorough study of the Permian Basin stratigraphy based on regional seismic lines or scattered wells could establish a better understanding of the basin sequence stratigraphy and possibly determine some of the key factors that lead to the deposition of the various strata. Currently, there are a vast number of studies related to the Permian Basin stratigraphy, but most are focused on specific areas or strata for the basin. There are but a few studies that provide an overall model for the whole basin and attempt to "put all the pieces together". The same problem was encountered in the attempt to find data related to the whole of Permian Basin hydrocarbon accumulation. Most of the public domain data is outdated and based on the 1979 USGS 79-838 compiled by Dalton et al.

Wednesday, February 13, 2013
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