Introduction of the Hanifa Formation

This section describes the high frequency sequence analysis of the 500 foot thick Hanifa formation (seal, source, and reservoir). This was undertaken to provide a better understanding of the configuration of the Arabian basin, its depositional history, leading to the development of a better reservoir model from which hydrocarbon extraction might become more efficient. Production engineers should be able take advantage of the identified fluid layers when planning production strategies.

Geological Setting
The late Oxfordian to early Kimmeridgian Hanifa source Rock was deposited in the intrashelf basin of the differentiated Arabian basin. These deposits are mostly low-energy, laminated, dark, organic-rich lime muds deposited under anoxic bottom-water conditions. To the north of this intrashelf basin, high-energy, shallow-water grainstones and evaporitic peritidal sediments accumulated across the Rimthan arch (McGuire et al., 1993). To the east, the continental margin separated the platform from the open ocean in which little or no deposition took place (Murris, 1980).

During the Hanifa Fformation deposition, the Arabian basin transition from the northern shelf-margin into the southern intrashelf sag was gradual. The northern margin of the basin by the Rimthan arch was characterized by shallow-marine oolitic and peloidal grainstones. Koepnick et al. (1994) indicated that there were indications of peritidal conditions and islands developing along the northern transition between the Rimthan arch and the Gotnia basin to the north. The basin gradually and gently dips to the southwest (slope < 0.5 degrees) forming a ramp-like depositional surface with a deeper gentle intrashelf depression to the southwest. This is indicated by a gradual southwestward thinning of the carbonate deposits and thickening of the evaporite deposits (salinas) in later regressive and restrictive stages. The Hanifa formation is overlain by the Lower Kimmeridgian shallow-water Jubaila formation.

Intrashelf Basin Characteristics and Development

Climate
Throughout the Late Jurassic, the Arabian basin was characterized by a very high rate of carbonate productivity in which a high organic content was preserved in the deeper anoxic regions of the basin. This high productivity has been attributed to the arid climate and a major wind direction from the southeast, which concentrated organic material in a narrow seaway (Irvine et al., 1974).

How intrashelf basins form
There are many explanations for the formation of intrashelf basins, but most of these basins appear to form on the shallow passive stable margins of plates. Here these shallow margins are extremely susceptible to the effects of marine transgression, particularly since in such settings tectonic subsidence tends to be minimal and the resulting sedimentary fill geometries are largely the products of sea-level variations (Aigner et al., 1989). Often these intrashelf basins develop as a result of a rapid eustatic sea level rise in which carbonate margins build up around an isostatically sagged deeper basin floor while the sedimentary fill lags at a slower rate of sedimentation (Read, 1985). As indicated above most intrashelf basins are short-lived and may be filled during the succeeding transgression event.

Intrashelf basin location and extent
The Qatar Arch separates the Arabian basin from the Rub' Al-Khali basin to the south (Southern Gulf basin of UAE and Qatar). The Rimthan Arch separates the Arabian basin from the Gotnia basin to the north (Kuwait and Iraq). During the upper Oxfordian and lower Kimmeridgian, these intrashelf basins were anoxic and the site of the accumulation of low-energy, organic rich lime mud (McGuire et al., 1993).

Previous work

Ayres et al. (1982) recognized that the upper Middle Jurassic source Rocks were deposited in an anoxic intrashelf basin separated by a grainstone and dolomitized facies from the open-marine environment of the Neo-Tethys Sea to the east. This intrashelf basin most probably resulted from the differential build-up of grainstones that kept up with a eustatic rise in sea level at the margin of the isostically sagged region. The high salinity conditions on the floor of this intrashelf basin aided the preservation of the organic matter.

Aigner et al. (1989) concluded that the intrashelf basin in which the Hanifa formation was deposited was the result of a rapid eustatic rise in the sea level across a stable tectonic region (the passive margin of the Neo-Tethys sea). They contended that the role of tectonic subsidence in the development of the Cratonic basin was minimal and the role of sea-level rise in the deposition of carbonates on the margins of an isostatically sagged platform interior was large. These intrashelf basins are different from the intra-Cratonic basins and tend to be short-lived and may have been filled during the subsequent seal-level cycle.

Droste suggested that the Hanifa intrashelf basin in which source Rocks were deposited was formed by a combination of load-induced isostatic sagging of the platform interior accompanied by a major rise in eustatic sea level.

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McGuire et al. (1993) provided a sequence stratigraphic depositional framework for the Hadriya and the Hanifa reservoirs in the vicinity of the Berri Field at the margin of the intrashelf basin to the south. Each of these shallowing-upward reservoirs is bounded by a drowning transgressive surface (ts) and the bulk of each reservoir Rocks consists of the lower aggrading and prograding grainstones of a highstand systems tract. The highstand systems tract (HST) is overlain by a sub-aqueous eroded surface (type-2 sequence boundary), which is followed by basinward progradation of a grainstones shelf margin wedge systems tract. The transgressive systems tract is identified by the back stepping grainstones onlapping the shelf-margin wedge systems tract (SMWST). The high-energy and shallow water setting in which the porous grainstone reservoir facies collected characterizes the regions in which these sediments were deposited. The deposition of both Hadriya and Hanifa reservoir sediments appears to have taken around 4 million years (147 Ma. to 143.5 Ma). Kendall et al. (1991) also concluded that major source Rocks were most likely formed during a rapid sea-level rise (transgression) as is the case of the Hanifa source Rocks; while best reservoirs deposits are associated with keep-up systems in which the sheet-like carbonate accumulations matched relative sea-level rise and aggraded to form shoaling upward cycles. Koepnick et al. (1994) in a study of the upper Tuwaiq Mountain formation concluded that the best reservoir facies were deposited within a highstand systems tract (HST) as aggrading and prograding grainstones.

Framework that houses sequence stratigraphy
The depositional history of the Middle and Upper Jurassic formations of eastern Arabia is best understood when described using a sequence stratigraphic hierarchy that involves the higher order (3rd and 4th order) sequences and their position within the lower-order (1st and 2nd order) sequences.

Droste (1990) identified two 3rd order sequences comprising the Hanifa formation during the transgressive phase of the 2nd order supersequence . For each of these 3rd order sequences, source Rocks were deposited in the intrashelf basin within transgressive systems tracts (TST) while reservoirs were deposited on the shelf margin within the highstand systems tracts (HST).

The 3rd order sequences corresponded to the time in which the Hanifa source Rocks were deposited and coincided with the maximum transgression of the Upper Jurassic 2nd order supersequence. A total of seven major reservoir horizons accumulated in the progressively more regressive 3rd order sequences within the late Middle Jurassic and Upper Jurassic formations (Koepnick et al., 1994).

The prolific deposition of regressive shallow shelf carbonates of the overlying Jubaila and the Arab formations fill the intrashelf bathymetric lows and the margin again became a carbonate ramp toward the end of the Jurassic.

Study Materials

Depositional Lithofacies
The Hanifa formation in the study area is comprised of eleven lithofacies originally identified by Saudi Aramco geologists from cores and used to construct a 3-D geological model that functioned as a template to control subsequent numerical modeling. Each lithofacies is characterized by specific porosity and permeability ranges, which could be utilized to predict the porosity and permeability distribution throughout the reservoir (Koepnick et al., 1994). Of the eleven identified lithofacies, the lime mudstone lithofacies 7 and 7.1 are non-reservoir, while the remaining nine lithofacies are reservoir facies. Generally, lithofacies could be used as an indicator of specific water depth and provide a key to the depositional setting in which it accumulated. Koepnick et al. (1994) identified forty-five geologic layers based on gamma ray and porosity markers that were correlated to 142 Berri Field Hanifa penetrations.

Outer Ramp to Upper Ramp-Margin Lithofacies
Shallow water, high-energy lithofacies (skeletal conglomerate, skeletal intraclasts, and boundstones) are most prominent in the outer ramp and upper ramp margin (north-most) and represent the best reservoir facies. These lithofacies were deposited just below the sea level and extend to a depth just below the fair-weather wave base.

Lithofacies 2 - Skeletal/peloidal/intraclast grainstone-conglomerate

Lithofacies 2.1 - Skeletal/peloidal/intraclast packstone-conglomerate

Lithofacies 3 - Massive skeletal/intraclast grainstone

Lithofacies 3.2 - Cross-Bedded skeletal/intraclast grainstone

Lithofacies 3.3 - Burrowed skeletal/intraclast grainstone

Lithofacies 8 - Stromatoporoid/coral boundstone/rudstone without mud matrix

Lithofacies 8.1 - Stromatoporoid/coral boundstone/rudstone with mud matrix

Middle Ramp-Margin Slope lithofacies

To the southwest, shallow-water facies give way to margin slope lithofacies (skeletal packstone and wackestone) that represent intermediate-quality reservoir facies. These lithofacies are believed to have been deposited above the storm wave base and below the fair-weather wave base.

Lithofacies 5 - Skeletal packstone

Lithofacies 6 - Skeletal wackestone/mudstone

Basinal lithofacies

Further to the southwest there are the non-reservoir, dense lime mudstone basinal lithofacies that serves as source Rock for the overlying reservoirs or seal to the underlying reservoirs. These basin lithofacies were deposited in quiet anoxic water conditions as suggested by the organic-rich lime mudstone of this lithofacies. The lamination indicates a weak current in a not too deep-water depth (Droste, 1990).

Lithofacies 7 - Organic-rich laminated lime mudstone

Lithofacies 7.1 - laminated lime mudstone

Droste's (1991) study found that this lithofacies has a total organic carbon (TOC) that amounts to less than 1% weight.

Correlation

The high-energy reservoir skeletal grainstones can be correlated mainly on the basis of lithofacies determined from core description, since the gamma ray log character provides no marked signal because the oxidizing conditions of the depositional setting removed the organic matter which would have concentrated the radioactive elements (McGuire et al., 1993). Shoaling upward parasequences and porosity logs were used to correlate the shallow parts of the reservoir (see later discussion of parasequences) while gamma logs were used for correlating the slope and basinal sections.

After experimenting with various datums (reference surfaces to hang or to draw all surfaces from), it was concluded that the best stratigraphic datum to be used for correlation was the bottom transgressive surface that separates the Hanifa sequence from the Hadriya sequence below. The maximum flooding surface forms a good time line, but when it is identified, it follows the profile of the basin and is not horizontal. A reconstructed sea level surface is the ideal horizon. This datum can be constructed by identifying all time-equivalent lithofacies from all parts of the basin.

Three major types of surfaces were used for correlating the whole section. These are the bottom and the top transgressive surfaces bounding the Hanifa reservoir, the maximum flooding surface, and the shelf-wedge margin sequence boundary (SB-2). These surfaces represent time lines (are chronostratigraphic) and separate older Rocks below from younger Rocks above. sequence boundaries (SB) formed when there was a lowering of the relative sea level and there was a reduction of accommodation space to a minimum. A SB separates the highstand systems tract below from the lowstand systems tract (or in this case, shelf margin wedge systems tract) above. On the other hand, a maximum flooding surface (mfs) coincided with a maximum rate of rise in relative sea level and an increase of the accommodation space to the maximum. The mfs separates the transgressive systems tract below from the highstand systems tract above.

I. Shelf-margin wedge (SMW) sequence boundary, type-2 (SB-2)

An SB-2 is identified by a sudden Gamma Ray spike (high), which corresponds to an increase in porosity in the shelf margin to basin region. The SB-2 converges into a very thin interval in the upper ramp margin and can be recognized only by the trend of the lithofacies in the succession. According Kendall et al. (1991), shelf margin wedges aggrade upward when the sea level rises slowly causing the continual carbonate production and the margin to prograde seaward in response to the lack of accommodation space. Goldhammer et al. (1991) noted that a SB-2 develops during a time of maximum rate of long-term accommodation decrease and marks a turn around from progressively thinning upward cycles into thickening upward cycles (a SB-2 is identified on the basis of on the character of stacking patterns including cycle thickness, facies character, lateral geometry, and early diagenetic attributes) (Goldhammer et al., 1991).

II. The maximum flooding surface (mfs)

An mfs is identified in the section by a sudden gamma ray spike (see cross-section) on the basinal wells. However, an mfs becomes increasingly difficult to identify toward the shelf since only shallow shelf facies and no shales are present; never the less, the gamma ray spike can still be recognized. The best means of identifying the mfs within shallow part of the basin (outer ramp) is by identifying a definite change in stacking pattern in which the cycles change from deep facies (wackestone and packstone dominated) into shallow facies (skeletal intraclasts). This geometrical trend can also be observed by laterally moving shelfward.

III. Transgressive surface (ts)

The upper transgressive surface which separates the Hanifa formation from the overlying Jubaila formation is recognized by a uniform increase in gamma ray signal, an increase of sonic velocity, and a lowering of porosity. All these effects are a response to a sudden deepening (increase of relative sea level).

Parasequences; parasequence sets; and systems tracts

The figure above illustrates some typical parasequences found throughout the section with their predicted depositional settings and occurrences within the sequence.

Shoaling-upward parasequences terminated by rapid sea level rise or a maximum flooding surfaces (mfs) (or using Kendall et al., 1991 terminology, give-up surface) are the basic genetic recognizable units in the section. The parasequences patterns and thickness are influenced by sea level fluctuation, rate of carbonate production, tectonic rate of subsidence, and subaerial exposure. No evidence of this latter was found in this section since it is believed that the shallower water sediment was being eroded and carried down slope. The parasequences are most useful here to detect relative-sea level fluctuation and to deduce a relative sea level on local scale and so then to be used for regional correlation (see discussion of Fischer Diagram below). parasequences also provide a key for confirming the major surfaces which cab used for correlating the shallow shelf section where the gamma log character is most ambiguous. For example, when a deeper lithofacies (i.e. lithofacies 5: skeletal packstone) overlies the shallow parasequences of shelf, then this can be used as an indication of sudden increase of the sea level and the initiation of the transgressive systems tract (TST).

Highstand Systems Tract (HST)

The parasequences thickness and composition is controlled largely by the deposition profile of the basin (in this case a gentle ramp). As a result, the aggrading-prograding parasequences of the HST thin toward the basin and gradually become indistinguishable as the section becomes mostly lime mudstone (forming a condensed section). On the other hand, HST parasequences are most easily distinguishable in the shallow region (north) of the study area where shallowing-upward cycles are capped by maximum flooding surfaces and sudden lithofacies changes from a shallow to a deep lithofacies. Consequently, the HST parasequences form the bulk of the reservoir in the north and center of the Hanifa formation and extend to center of the formation. These parasequences downlap on the mfs basin-ward and are truncated by the sequence boundary on the shelf.

Shelf-Margin Wedge (SMW)

The shelf-wedge margin systems tract (SMW) is bounded below by a type 2 sequence boundary (SB-2) and is identified by prograding shallow shelf lithofacies over the subaqueous sequence boundary. The prograding parasequences (layers) converge (thin) where they collect towards and on the shelf and thicken toward the basin. The central wells provide the clearest aggrading parasequences of the SMW and are used here for correlation throughout the field. The SMW parasequences onlap on the sequence boundary (SB-2) with thicker laminations (deposited within shallower water) in the basin and thin toward the shelf edge. The sequence boundary is recognized indirectly on the basis of a basin-ward shift of the lithofacies. The parasequences change in composition from deep lithofacies into progressively shallow lithofacies with thinner and thinner packstone and wackestone lithofacies in the shelfal parasequences with more and more skeletal intraclasts and conglomerates occurring within them. The basinal parasequences also experience the same gradual change as the relative sea level shallows and accommodation space decreases. The basinal parasequences also gradually lose the lime mudstone lithofacies composition to more packstone and wackestone and even skeletal intraclasts in the SMW. As a result, the SMW parasequences form the upper portion of the reservoir in the southwest region of the Hanifa formation.

Transgressive Systems Tract (TST)

Sea Level Re-Construction from lithofacies succession

Fischer plots were used successfully to extract the 3rd-order and higher sea level fluctuations from high frequency cycles identified on the section. They illustrate the long-term accommodation changes similar to those described by Goldhammer et al. (1991) and "reveal systematic changes in accommodation by plotting successive deviations in cycle thickness from the average cycle thickness" (Read and Goldhammer, 1988). Fischer diagrams (plots) have been constructed from two wells to determine the feasibility of constructing the third order sea level curve and then comparing it to the Haq et al (1987) sea curve to establish a match. Another important objective was to use the plots to discern the role of subsidence (when plots of various parts of the region are compared). The selected wells represent the shallowest part of the section (well # 014) and the center part of the section (well # 006). Selecting wells from the deep part of the section was not useful since identifying parasequences based on lithofacies succession was not possible because the lithostratigraphy consisted only of lime mudstone.

Three fourth-order sequences were identified after constructing the Fischer diagrams. 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. Each of the 4th order sequences has a recognizable maximum flooding surface (which corresponds to the 3rd order sequence mfs in one of these sequences). An important observation is that the maximum flooding surface (mfs) identified based on the recreated sea level curve (at the point of maximum cumulative cycle thickness point) has a different position from the mfs identified on the section. The first cycle of each Fischer diagram is very thick due to the fact that deposition started with thick basinal cycle (lime mudstone) at the base of the section. However, both reconstruct 3rd order curve faithfully and represent the actual relative sea level fluctuation position that occurred when these cycles were deposited.

Conclusions
This analysis confirms the conclusions of previous work that the bulk of the Hanifa reservoir is composed of skeletal conglomerates which were deposited on the relatively high-energy shelf (northern region of the formation) within the highstand systems tract of the upper Hanifa sequence. We also found that the contemporaneous organic-rich lime mudstones were deposited on the southern region of the basin during late transgressive systems tract and during the highstand systems tract under quiet and anoxic water conditions. The lithofacies shift from the shelf to basin is gradual and grades through the identified eleven lithofacies in response to the changes of the relative sea level in which they accumulated. The deposition of these lithofacies was also greatly affected by the depositional profile of the basin, which was very gentle ramp with an intrashelf basin to the south. The lithofacies distribution in the basin enables the prediction of the best reservoir facies, mainly to the north, and the interpolation of the intermediate lithofacies. The Fischer diagrams enabled the reconstruction of the sea level in the peritidal shelfal regions and generally mimicked the Haq eustatic sea level curve. The Fischer diagrams also revealed three fourth-order sequence that were not identified earlier.

Future Work
A more in-depth analysis of the basin should be possible if more well data were made available. In such a 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 to the north.