An in-depth analysis gave Anadarko the confidence it needed to acquire acreage in the eastern Gulf of Mexico basin.

An understanding of the regional geological framework of the Eastern Gulf of Mexico was imperative to Anadarko Petroleum Corp.'s assessment of the hydrocarbon potential and technical risks associated with exploration within the boundaries of Federal Lease Sale 181. On Dec. 5, 2001, the Minerals Management Service offered industry 233 federal tracts in the DeSoto Canyon and Lloyd Ridge Areas,
which had been off-limits to leasing and exploratory drilling since 1988. The area, located in water depths ranging from 5,000 ft to 9,500 ft (1,525 m to 2,750 m), constituted one of the largest contiguous unexplored areas (approximately 1.5 million acres) in the Cenozoic part of the Gulf of Mexico Basin.

Study area

An analogous study area, consisting of eastern Mississippi Canyon, southern Viosca Knoll and northern Atwater Valley, provided the regional "model" for understanding the petroleum geology of the unexplored Sale 181 Area. This "study area" was in a mature stage of exploration with 100 exploratory tests and 40 field discoveries providing valuable information for the establishment of a regional framework that could be extrapolated into the undrilled Sale 181 Area.

Regional objective

The purpose of conducting a comprehensive regional analysis of the proposed study area was to:

• define the chronostratigraphy, depositional environment and facies type of the prospective reservoir rocks;
• understand the play styles and trapping mechanisms;
• confirm the existence of an active petroleum system and predict the hydrocarbon type and quality; and
• conduct a statistical analysis to define prospect success rates and potential reserve size distributions.

Methodology and interpretation

Stratigraphic Sequences. Correlation of key exploratory wells, some of which penetrated the entire Cenozoic section, combined with biostratigraphic and paleo-ecologic information, abundance and diversity curves, and 2-D seismic provided the basis for the definition of the major chronostratigraphic sequences in the Eastern Gulf of Mexico (Figure 1). A series of regional 2-D seismic transects, constrained by well control, were used to "correlate" the major sequences from the defined study area to the sale area.

Depositional Synthesis. Lithologic information (net sand, percent sand, sedimentation rates, individual sand thickness and vertical stacking patterns) was gathered for each chonostratigraphic sequence from all released well logs in the study area. These data, coupled with regional seismic amplitude extractions and conventional core interpretations from 14 fields, led to the development of depositional models and the understanding of sand distribution patterns and to the prediction of sand facies type (channels, levees, amalgamated sheets, etc). In areas of limited well control, sequence isopach mapping was used to predict slope paleo-topography in an attempt to delineate sediment transport fairways and predict locations most prone to turbidite sand deposition (Eisner and Mead, 2001).

Regional mapping indicated that middle and late Miocene fluvial-dominated delta complexes prograded from the northwest to southeast along the Florida Escarpment, providing the "staging area" for the sand-rich slope fan systems that encompass the major reservoir facies in the study area. A stable depositional substrate, due to the absence of allocthonous salt, resulted in predominately channel-levee complex reservoirs (Aconcagua and Horn Mountain Fields) within most of the study area. Frontal splay (lobate) reservoirs occur locally where accommodation space was created by erosional irregularities in the Paleogene depositional substrate and by ponding effects of pre-existing paleo-topography (Petronius and Marlin Fields). Regionally, 3-D amplitude extractions indicate a general pattern of channel-levee facies yielding to frontal splays in a north-to-south direction. Flow stripping, whereby the mud is progressively winnowed from the system, results in progressively smaller levees, gradually giving way to unconfined flow and the deposition of lobate sheet sands in the more distal parts of the slope fan system.
Trap Styles and Play Types. Hydrocarbon trapping styles consist of salt uplift features, stratigraphic embayment pinchouts and stratigraphic shale-outs on monoclinal dips (Wilson, et al., 2001). Anadarko conducted in-house detailed field studies of at least one analogous field from each of these end-member play types. Comparative field studies (integrating 3-D seismic, well log suites, paleontology, conventional core interpretations and geochemical information) are an integral part of any regional synthesis. Understanding the structural and stratigraphic controls of commercial hydrocarbon accumulations allowed for a better technical evaluation and risk assessment of our Sale 181 prospect inventory.

The "stratigraphic embayment" play type is exemplified at Petronius Field (100 million boe) where Middle Miocene sandstones onlap the Florida Escarpment, which marks the eastern edge of the Cenozoic Basin. The Petronius reservoirs are interpreted to have been deposited as "frontal splays" related to a Middle Miocene (MM7) submarine fan system. The lobate geometry of the sand body is partially controlled by paleo-topographic relief related to submarine erosion and mass wasting along the buried Florida Escarpment.

Ram-Powell (275 million boe) provides an analog for a traditional stratigraphic hydrocarbon accumulation where Middle Miocene (MM9) reservoirs are trapped by a gradual facies change from proximal levee sands to distal levee silts to overbank muds (shales) along monoclinal dip. The levee-overbank environment at Ram-Powell proved to contain excellent reservoirs, as completions in proximal levee sands have produced in excess of 15 million boe/well (Clemenceau, et al., 2000).

Additionally, Mesozoic play types associated with deeper salt uplift and withdrawal features were recognized in the Lower Cretaceous and Jurassic clastic sections (Denman & Adamick, 2000). The stratigraphic information attained from the drilling of the MC 392 just prior to the lease sale coupled with the lower-risk profile of Cenozoic objectives led Anadarko to target Miocene salt uplifts and stratigraphic traps as primary objectives in the Sale 181 Area.

Hydrocarbon Quality. Analyses of produced hydrocarbons from the defined "study area" yielded variable information; thermogenic gas, oil and biogenic gas were all reported from Miocene reservoirs. Published information suggested that Jurassic carbonate-sourced hydrocarbons, resulting in low-quality (low-gravity, high-sulfur) crude, were common in fields in the western part of Mississippi Canyon. To the north (southern Viosca Knoll and northern Mississippi Canyon) the produced oils were reported to be of much better quality, and a Mesozoic shale source was suspected. A well at King (MC84) penetrated thermally immature but high-quality shale source rocks within the lower Tertiary and Cretaceous sections (Wagner, et al., 1994).

The lack of drilling within the Sale 181 Area, along with variable hydrocarbon quality in the adjacent study area, resulted in a degree of uncertainty as to the potential hydrocarbon type and quality that might be expected in the sale area. A complex hydrocarbon system was suspected, leading Anadarko to the acquisition of seafloor piston core samples on a regional grid over prospective areas, and along "migration pathways" above known hydrocarbon accumulations.

Geochemical analysis of these piston cores indicated an active petroleum system with thermogenic hydrocarbon encountered, particularly along the Florida Escarpment, where migration pathways extended to the ocean floor. Mesozoic shale-prone source rocks were inferred from the results of the biomarker studies (Chaouche and Crews, 2001).

Thermal modeling results indicate that a component of lateral migration was necessary to explain Mesozoic-sourced hydrocarbons in the Sale 181 Area. In addition, geochemical and thermal modeling results suggested the higher probability of oil reservoirs in the southern part and mixed oil/gas reservoirs in the central and northern part of the Sale 181 Area (Chaouche and Crews, 2001). The origin and distribution of the biogenic methane was problematic and less predictable; it ranged from a trace to nearly 100% of the total hydrocarbons in the study area fields.

Statistical analysis

Since 1985 industry drilled 100 new-field exploratory tests yielding 40 commercial fields for a 40% success rate. Operators have disclosed their estimated ultimate recovery (EUR) on 36 of these 40 fields yielding reserves of 4.6 billion boe. Mean and median field size for these announced accumulations are 128 million boe and 76 million boe, respectively. Upper and Middle Miocene sandstone reservoirs account for 88% of these discovered hydrocarbons.

For the 29 discoveries where paleontologic data are available for age definition of the hydrocarbon-bearing section, the Middle Miocene (MM9) chronozone is by far the most prolific. This stratigraphic interval, ranging from 9.9 million years ago (mya) to 12.8 mya, includes the primary pay sands in 19 of these fields and contains 58 % (1.35 billion boe) of the total announced reserves.

Summary

Regional studies played an integral role in the technical assessment of Anadarko's Sale 181 Prospect Inventory. The study assisted Anadarko's Eastern Gulf of Mexico exploration team in placing the Sale 181 prospects within a regional petroleum system framework. The study defined: (1) the age, depositional environment and facies type of the prospective reservoir rocks; (2) the play types and trapping styles; (3) the expected hydrocarbon quality and the existence of an active petroleum system; and (4) the prospect success rates and most probable reserve size ranges.
Anadarko was one of the most aggressive bidders in Sale 181, submitting high bids totaling US $136 million for the acquisition of 26 tracts (approximately 150,000 acres). Exploratory drilling was scheduled to begin in March 2003.

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