US Gulf, Atlantic deepwater fans hold high potential - MMS assessment

Beyond the Sigsbee Escarpment

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Gulf of Mexico reserves discovered by year.
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A study conducted by the US Minerals Management Service covering the US Gulf of Mexico and Atlantic OCS leasing areas delineated 72 plays, of which 52 comprise the Gulf of Mexico Cenozoic Province, 9 comprise the Gulf of Mexico Mesozoic Province, and 11 comprise the Atlantic Mesozoic Province.

These 72 plays account for a mean total endowment of nearly 26 billion bbl of oil and 300 Tcf of gas. Of this mean total endowment, 11 billion bbl of oil and 123 Tcf of gas are undiscovered resources. Moreover, nearly 80% of these undiscovered resources are projected to occur in Gulf of Mexico plays, with 8 billion bbl of oil and 96 Tcf of gas. The study was entitled "Assessment of Conventionally Recoverable Hydrocarbon Resources of the Gulf of Mexico and Atlantic Outer Continental Shelf as of January 1, 1995."

Deepwater potential

With over 1,000 oil and gas fields, the Gulf of Mexico has often been considered a mature basin. However, the vast majority of these fields are on the heavily drilled shelf. The deeper-water area (>200 meters) is even larger than the shelf, has been only lightly explored, and has only recently begun to have a significant impact on US Gulf production.

It is predicted that deepwater oil production will soon surpass shallow water oil production. Extending basinward even farther, the ultra-deepwater area (>2,000 meters) has several recent, very large discoveries. Some of these discoveries represent extensions to existing plays, but others represent new plays that are just beginning to be explored and can be considered as frontiers.

The geologic setting in the deepwater US Gulf is favorable for the existence of significant hydrocarbon accumulations, and this has been proven in recent years with the discoveries of deepwater fields such as Diana (East Breaks 945), Baldpate (Garden Banks 260), Troika (Green Canyon 244), and Ursa (Mississippi Canyon 810).

The deepwater area has active salt tectonics that create abundant traps and sand fairways out to and beyond the Sigsbee Escarpment and has underlying mature Mesozoic source rocks. Evidence of mature source rocks in the deepwater area includes:

  • Ocean bottom vents expelling gas and oil
  • Natural oil slicks seen at the ocean's surface by aircraft and satellite photos.

The results of this assessment reflect the immature/frontier nature of the deepwater area. Comparing the mean total endowment by depositional style, deepsea fans are estimated to contain by far the most undiscovered resources, with almost 16 billion BOE, which is 62% of the total. This is more than four times the amount contained in progradational sediments, the most prolific producers on the shelf.

Deepsea fans

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Pool ranking by reserve size: discovered pools in red, undiscovered in blue.
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With the most exploration potential in the Gulf of Mexico established in deepwater fans, each fan play can be compared to narrow down areas of better exploration potential. The

Lower Pleistocene fan (LPL-F) and the Upper Pleistocene fan (UPL-F) plays are estimated to contain the most undiscovered resources of the 13 established fan plays, with 3.0 and 2.7 billion BOE, respectively. Furthermore, within the established reserves and hydrocarbon limits of a play, there are areas of better reservoir sand quality. This is useful in projecting the presence of good sand potential downdip of these areas.

For example, by examining established trends for the LPL-F play, such as average pay thickness and porosity, the best sand development can be inferred to exist in central and eastern East Breaks, eastern Garden Banks, eastern Green Canyon, and Mississippi Canyon. Additionally, the geologic and engineering data indicate that the LPL-F play appears to become more oil prone downdip. Another factor enhancing the exploration potential of the LPL-F play is the possibility for encountering hydrocarbons in shallower plays. The Middle Pleistocene fan (MPL-F) and UPL-F plays extend over much of the same area as the LPL-F play.

Data, methodology

As a condition of exploration permits and leasing activities, MMS has acquired approximately 1.2 million line-miles of two-dimensional common depth point (CDP) seismic data and 28,000 square miles of three-dimensional CDP seismic data. Moreover, MMS has accumulated geologic information, including all well logs, paleontological reports, and oil and gas production data, from over 31,000 wells drilled on the Gulf of Mexico and Atlantic OCS.

At the time of this study, 1,096 OCS fields (876 proved, 77 unproved, and 143 expired with no production), containing 22,172 reservoirs in 8,856 sands were present in the Gulf of Mexico.

A framework for organizing MMS's extensive geologic and reservoir engineering files associated with these fields was needed. A play assessment framework was judged to be the best approach to meeting this objective and represents a major change from the procedures used by MMS for previous assessments.

A play is defined as a group of reservoirs genetically related by depositional origin, structural style or trap type, and nature of source rocks or seals and forms a natural geologic population limited areally and stratigraphically. Once divided into plays, all reservoirs within a particular play will have production characteristics that are more closely related than characteristics of reservoirs in other plays. Better known reservoirs can have their attributes extrapolated to lesser-known reservoirs.

The play analysis approach provides for the systematic documentation, integration, and analysis of the play's geologic model and exploration history and an assessment of the size and number of undiscovered hydrocarbon accumulations.

Hydrocarbon endowment

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Total hydrocarbon endowment based on play type.
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The principal purpose of this study was to present estimates of the total endowment of conventionally recoverable oil and gas that may be present beneath the Gulf of Mexico and Atlantic OCS in water depths as great as 3,000 meters. Secondary objectives were to describe the geologic and mathematical methodologies employed in the assessment, present an economic analysis of the undiscovered conventionally recoverable resources of the area, and provide a historical perspective in which to review the results.

The geologic, reservoir engineering, and production data included in this assessment, when combined with the data and information previously released, provide explorationists and resource analysts with a wealth of publicly available data to perform resource assessments or evaluate the economic viability of the resources.

For each assessed play, a geologic description, reservoir characteristics, discovery history, reserves and appreciation, and cumulative production have been provided. Additionally, the play's resource potential is portrayed as a pool rank plot identifying both discovered and undiscovered pools.

This same information also is presented at the chronozone, series, system, province, region, and margin levels. Additionally, various geographic information system (GIS) data are provided, including play limits; coastal and Federal-state boundaries; protraction areas and blocks; field and pool outlines; pipelines; platforms; planning areas; generalized bathymetry; fairways; royalty relief lines; and Mesozoic well data.

Play maps as tools

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Lower Pleistocene play limit and production.
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Hydrocarbon accumulations in the northern Gulf of Mexico are placed into plays based on geologic age (chronozone) and large-scale depositional patterns (facies). Three distinct limits were constructed for each established play. The reserves limit includes all active fields with proved reserves and selected unproved fields that were deemed to be economically viable at the time of this assessment. Next, the hydrocarbon limit was determined by correlating hydrocarbon-bearing wells outside of the reserves limit to the proved and unproved fields within the reserves limit. Finally, the play limit was determined by correlating non-hydrocarbon-bearing wells with sand outside of the hydrocarbon limit to the fields and wells within the hydrocarbon limit.

The general progression from established reserves and production within the reserves limit, to known hydrocarbon occurrences within the hydrocarbon limit, to known sand occurrences within the play limit, is observed for each play. This progression can be used in a general sense to extrapolate hydrocarbon trends and as a play occurrence guide.

For example, within a chronozone, a typical facies assemblage comprises downdip deepsea fans toplapped by progradational sediments, which in turn are overlapped by nearshore aggradational deposits. These facies do not always include productive sands, but all are usually present in each chronozone in the Gulf of Mexico.

If production has not been established from one of the facies, then it is assumed that the facies is present, but has not yet been adequately drilled or proved to be productive. Other examples of play-based exploration are the deepening of wells in fields where a known producing horizon has not yet been reached, testing undrilled structures within a producing trend, and drilling wildcat wells beyond current productive limits.

Pool rank plots as tools

Exploration companies are typically interested in estimating the total remaining undiscovered hydrocarbon potential of a play and the probability of an accumulation exceeding a minimum threshold size. A play's historical performance, related to the number and size of discovered pools, fit to a mathematical model (log-normal distribution), allows extrapolation into the future in the form of pool rank plots.

For a particular play, information such as the number of discovered and undiscovered pools, their sizes, and whether the largest pool in the play has been discovered provides quantitative answers to typical questions. Additionally, for conceptual or immature plays, an exploration company can decide whether the play is likely to contain enough resources to warrant continued exploration effort.

On the presumption that the remaining undiscovered hydrocarbon potential of a play has passed company screening criteria, play-specific and pool-specific geologic and reservoir engineering data in the study can be used to calculate directly the resource potential of a mapped prospect. The values of selected pool-specific data parameters (total productive area, average pay thickness, average water saturation, average porosity, and others) can be plotted as frequency density distributions that can be used directly or as a guide, in conjunction with prospect-specific information, to in assess an individual prospect's hydrocarbon potential.

The play's pool rank plot can then be used to judge the reasonableness of the prospect evaluation and the relative probability of a pool of the estimated size remaining to be discovered in the play.

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