EXPLORATION U.S. CONVENTIONAL RESOURCES-2

April 14, 1997
John D. Grace Earth Science Associates Arlington, Tex. Highly Mature Plays with Potential* [14238 bytes] Top 6 Plays in Gulf of Mexico* [69335 bytes] The 1995-96 National Oil and Gas Assessment by the U.S. Geological Survey and U.S. Minerals Management Service analyzed undiscovered hydrocarbon liquid and gas potential of more than 500 plays.

Resource assessment analysis identifies 21st century plays

John D. Grace
Earth Science Associates
Arlington, Tex.
The 1995-96 National Oil and Gas Assessment by the U.S. Geological Survey and U.S. Minerals Management Service analyzed undiscovered hydrocarbon liquid and gas potential of more than 500 plays.

This second of three articles examines those plays which, measured by several standards, might be called the "top" plays in the country. These are the plays that could be expected to make the largest contributions to newly discovered oil and gas in the next century.

There is no singly correct way to rank exploration plays. Many factors enter into a company's comparisons beyond the estimated total volume of undiscovered oil and gas in the play as a whole. Other determinants of attractiveness include range of prospect sizes, depth, exploration dry hole risk, production and cost characteristics, location, and technology. Nevertheless, plays which combine large undiscovered volumes, high average field size and very large maximum field size are always alluring.

Not surprisingly, just as most discovered oil and gas are concentrated in a few, very rich plays, so most of the nation's estimated undiscovered hydrocarbon volumes are similarly localized.

These top plays owe their status to various causes.

Some represent "tail-end" prospects in what have already been extraordinarily prolific plays.

Some are the geologic extension of proven ideas to new environments within a larger, established petroleum system.

Some are hypothetical plays in areas of little exploration.

Finally, some plays owe their large remaining endowments to having been placed off limits, beyond the reach of most exploration activity.

The top plays are presented in three divisions.

First is Alaska; it is unique in richness and expected field size and also in the economics of exploration, development, and transportation.

Second are the leading onshore, Lower-48 plays.

Last are the top plays offshore the Lower 48 states, concentrating on the Gulf of Mexico. While offshore domains are generally extensions of onshore geology, their different exploration maturity and economics warrant separate treatment.

Alaskan plays

Alaskan exploration potential, both on and offshore, is concentrated in the arctic ( Fig. 6 [89454 bytes] and Table 2 [29679 bytes]).

Plays of the Alaskan arctic can be grouped based on the tectono-stratigraphic evolution of the region. The deepest prospective section is the Ellesmerian, from Mississippian through Triassic age. Next are the rocks associated with the opening of the Beaufort and Chukchi seas during Jurassic and Early Cretaceous time (referred to here as Beaufortian). Finally, there is the Brookian section.

This last cycle, extending from the Lower Cretaceous to the surface, is a northerly prograding system of clastic units derived from the thrusting of the Brooks range in the south.

The highest undiscovered potential is in the Brookian sequence. The Brookian orogenic belt, the principal source of reservoir sediments, defines the southern margin of the province. Northward verging thrusting started in the west in Jurassic time and spread east, then north, approaching the coast at the U.S.-Canadian border during Tertiary. The eastern extent of the thrust belt itself is estimated to contain the largest quantity of conventional, technically recoverable, undiscovered natural gas of the onshore U.S.

Although the structuring of the region and trap formation was Brookian in age, the reservoirs involved in the Eastern thrust belt lay are Beaufortian and Ellesmerian.

Immediately in front of the thrust belt are folded sequences that involve the entire Brookian section. Two plays are defined here: onshore and a separate offshore play, which continues into the Beaufort Sea north and east of the mouth of the Canning River.

Gas is expected to predominate in onshore, particularly in the western part of the play. Liquids content increases to the east, and estimated oil resources exceed gas offshore.

In front of the fold belt is a wide band of relatively undisturbed Brookian clastic sequences extending from the Chukchi Sea in the west to about the mouth of the Canning River in the east. It begins in the south at the northern limit of the fold belt, crosses the Barrow arch, which is roughly coincident with the coast, and continues for 50-75 miles north of the coast.

Both USGS and MMS made the first order division of the Brookian section into plays based on seismic-stratigraphic position. Topset plays are expected to contain more hydrocarbons and larger field sizes, particularly offshore to the east of the Colville River delta. Foreset and bottomset units were grouped both onshore and offshore into turbidite plays.

A further division was made by MMS offshore along the crest of the Barrow arch, based on the degree of structural involvement in trapping. South of the arch a set of unstructured plays is recognized; north of the arch they are referred to as faulted.

In all, the seven top Brookian plays in Table 2 are estimated to contain 9.5 billion bbl of liquids and 53.6 tcf of gas. This is about a third of total arctic conventional, technically recoverable, undiscovered resource. They also contain most of the largest exploration targets.

The hydrocarbon potential of the section has been demonstrated onshore by a number of small, conventional fields and Brookian pools within larger fields. The presence of large volumes of hydrocarbons is shown by the billions of barrels of oil in place at the supergiant heavy oil accumulations at Ugnu and West Sak near Prudhoe Bay.

Offshore, Brookian reservoirs have tested hydrocarbons in a number of exploratory wells, although details have not been released. Some of the offshore plays have not been proven and are identified as hypothetical.

After the Brookian section, four other top plays are scattered geographically and stratigraphically in the Alaskan arctic. The first two are in the Beaufortian section on the Barrow arch and in the Chukchi Sea (rift-active margin play). Third is in the Mississippian Endicott formation on the North Chukchi platform. Finally, there is the eastern extent of the Brookian thrust belt, mentioned above.

Lower 48 land plays

USGS, in its assessment of conventional, undiscovered hydrocarbons onshore and in state waters, defined about 470 plays.

A simple sorting of the plays by estimated mean size of remaining undiscovered field yields an important first cut. The top oil and gas plays, measured by this criterion, are shown in Fig. 7 [24811 bytes].

Two interesting groups emerge. First, in the thrust belts of the northern Rockies and immediately in front of them are significant opportunities for large discoveries of both oil and gas. Second are the stratigraphically down- dip and deeper extensions of plays in the Gulf Coast and Anadarko basins, which are key sources for new, large gas discoveries.

Rocky Mountain plays

Of the top plays in the northern Rocky Mountains, three are in the Wyoming thrust belt province (Fig. 8 [64956 bytes] and Table 3 [17397 bytes]). The northern thrust play is hypothetical. USGS proposes accumulations in Paleozoic reservoirs (Ordovician through Mississippian, most importantly the Madison limestone), where reservoirs are thrust over organically rich Cretaceous source rocks across the Crawford-Meade, Absaroka, and Prospect-Darby thrust systems. Based on estimated risked undiscovered oil and gas in the play (i.e., on a BOE basis), the northern thrust play is the richest hypothetical play in the Lower 48 onshore/state waters assessed by USGS.

The Moxa arch extension play projects the Paleozoic reservoirs of the highly-explored Moxa arch-LaBarge play north, beneath the easternmost thrust system of the Wyoming thrust belt, the Hogsback. It is a narrow (6-10 miles wide and 80 miles long) band north from LaBarge through eastern Sublette, western Lincoln, and into southern Teton counties, Wyo. As footwall anticlinal traps, the same units that are productive to the south are estimated to contain CO2-rich gas fields.

Almost all the discovered hydrocarbons in the Wyoming thrust belt province are in the Absarokan thrust play-249 million bbl of oil and 1.3 tcf of gas. Reservoirs in the hanging wall range from Ordovician to Cretaceous age, with the most important discoveries to date being the Jurassic Nugget formation.

Over much of the thrust plane, hanging wall reservoirs are in direct contact with rich organic Cretaceous shales in the footwall. In all three Wyoming thrust belt plays, difficult terrain, very complex structure, and degraded seismic information challenge exploration.

The two other plays of note in the region are to the north. At the western end of the Big Horn basin, beneath Eocene age volcanics, lie the Paleozoic through Mesozoic reservoirs of the Sub-Absarokan play. Because of the difficulty of exploration through the volcanic section, even though the sedimentary section is shallow (median reservoir depth is 3,000 ft), only a few wells have been drilled and six small fields found. However, estimated total oil resources and mean field size of 20 million bbl makes the play an important technology target.

Finally, in the Montana thrust belt there is the hypothetical imbricate thrust gas play. The principal target is the Mississippian Madison limestone. High gas potential is credited to the play principally because several large potential traps remain undrilled and gas is known to occur in the section.

Gulf Coast plays

The second large source of Lower 48 undiscovered potential is in the extension of established plays into stratigraphic environments beyond traditional areas. This is most clearly seen in the southern Gulf Coast onshore, involving the downdip extension of three established gas plays-the Upper Wilcox downdip, Yegua downdip, and the Deep Tuscaloosa (Fig. 9 [95063 bytes] and Table 4 [15618 bytes]).

All three plays are continuations downdip of the well-explored deltaic and more proximal environments across central Louisiana and coastal Texas to slope and fan facies beyond the paleo-shelf edge. In all cases, porosities and permeabilities are well preserved in discovered fields, but reservoirs are heterogeneous and overpressured. The productivity of all three is well established by discoveries, although, because of depth, economics play a critical role in exploration.

Of obvious interest in Fig. 9 is the wedge extending from south of Baton Rouge to east of Lake Pontchartrain, where all three plays overlap vertically. While there has been some drilling in this zone in the 1980s and early 1990s, it remains largely untested.

The last of the top plays onshore and in state waters of the Gulf Coast basin is the Mobile Bay Norphlet. In addition to the 5.26 tcf estimated in state waters, the play continues into federal waters as the Upper Jurassic play (see below). That part of the play is forecast to contain another 7.17 tcf of technically recoverable gas, making the combination one of the most prolific U.S. gas plays.

A broader look for the best plays would include more parameters. Beyond estimated field size range (mean to maximum), two more variables were added and the plays re-sorted. These variables were the total volume of undiscovered oil and gas in the play and a first order economic variable, unrisked barrels of oil or thousand cubic feet of gas (mcf) per dollar of drilling costs.

The parameter unrisked barrels or mcf per dollar was calculated by dividing the expected mean size of undiscovered oil or gas fields in the play by the cost of drilling in that play to the estimated median depth of undiscovered reservoirs.

Software to rank onshore and state waters oil and gas plays using several different parameters assessed by the USGS is available free on Earth Science Associates' World Wide Web page. The program is called Play Ranker 1.0.

The address is www.earthsci.com/~esa

High maturity plays

The most interesting group of plays to emerge from this fuller examination was that which is constituted by the tail-end of very prolific old plays. For these mature, rich plays, in percentage terms, only a relatively small share of the play's original endowment remains undiscovered (Fig. 8). However, these were enormously productive plays from the start.

For example, while all of the plays in Fig. 8 have less than 15% of their original endowment left as undiscovered resources, each still has estimated undiscovered technically recoverable resources from slightly less than 300 million to almost 1 billion BOE. Because they are in the middle of very well developed infrastructure, marginal costs are low and exploration is very attractive.

The most important plays in this class are in the Permian Basin. The clear standout is the San Andres-Clearfork play on the northwest and eastern shelves. These are principally low-permeability carbonates, with up to 1,000 ft of gross reservoir interval in a variety of trapping configurations.

The two largest fields, Wasson and Slaughter-Levelland, both discovered in 1936, together have produced over 5 billion BOE. The anticipated mean of future oil discoveries is 7.7 million bbl, which is very high for such a mature region.

Of the other three Permian Basin plays, two are also distributed across the northwestern and eastern shelves of the basin. They are the Pre-Pennsylvanian and Upper Pennsylvanian, both also highly mature.

The former play is composed of a principally of Ordovician and Mississippian carbonates, with porosity enhanced by dolomitization, fracturing, and karstification. The fields of the latter play are usually multiply stacked reservoirs of the Strawn, Canyon, and Cisco formations, sealed by interbedded anhydrite, shale, and siltstone members.

The third Permian Basin play is gas, limited to the Ordovician and Silurian of the deep parts of the Delaware and Val Verde basins. Drilling depths range from 8,000 to 24,000 ft.

The first discovery in the play was Puckett field, found in 1952, which had produced 2.9 tcf through 1990. The largest field is supergiant Gomez field, which has cumulative production of 5.3 tcf.

In the San Joaquin basin, two plays located in the structurally and stratigraphically complex fold belt on the west side of the basin rank highly. The older rocks are covered by the Pre-Middle Miocene source play, the younger rocks by the Post-Lower Miocene source play. They contain billions of barrels of discovered oil, some of it heavy, in giant fields found as early as the first decade of this century.

Because of the complexity of the fields and the plays, the distinction between extension/new pool exploration and new field exploration is blurred.

Last among the prolific mature plays is the Neogene section of the Ventura basin.

The final group of plays of special interest consists of those prospective areas in which exploration is prohibited or impeded by political or practical concerns. These include the extensions of the Niagaran reef play from onshore northern Michigan into Lake Huron and Lake Michigan; the state waters extension of the onshore Paleogene and Neogene plays of the Ventura basin, and the prolific plays which underlie the city of Los Angeles.

Collectively, estimates of technically recoverable undiscovered oil and gas in these three areas alone equal 2.8 billion BOE.

Lower 48 offshore plays

Although the MMS completed assessments for the offshore Pacific and Atlantic federal waters, the most important results pertain to its findings for the Gulf of Mexico. There, in analysis conducted in extreme stratigraphic detail, MMS identified several plays and types of plays which hold high promise.

The biggest surprise of the Gulf of Mexico assessment was the quantity of hydrocarbons MMS estimated to have remained on the shelf, in water less than 200 m deep. The reason behind this was that a large portion of those estimated undiscovered accumulations is postulated to exist in stratigraphically distal environments, mainly in the Pleistocene, which are deep objectives.

The top five plays in federal waters of the Gulf are shown in Fig. 9 and Table 5 [19368 bytes]. Collectively, these plays account for about half of the conventionally recoverable undiscovered hydrocarbon liquids and gas in the region.

The top two alone, the Upper Pleistocene and the Lower Pleistocene, contain between a quarter and third of undiscovered resources. The other Cenozoic play is the well-established middle Middle Miocene (MM7 in MMS terminology) off the Texas coast.

There are two Mesozoic plays. The Upper Jurassic is principally the Norphlet formation, which is an extension of the Mobile Bay Norphlet play in state waters. There have been six discoveries in state waters and nine in federal waters.

The other Mesozoic play is the Lower Cretaceous patch reef play, which has proved productive in seven wells in Main Pass 253 field and Viosca Knoll 252 field. Unfortunately, because of the way MMS defined play outlines, there is basically no information on where, beyond these fields, this play might extend.

Because of the stratigraphic detail of the MMS analysis, both the discovered and undiscovered resources of the Cenozoic section of the Gulf of Mexico were divided into groups based on the environment of deposition of the reservoir units.

The most important in terms of discovered resources is the "progradational" environment, associated with periods of basinward advances of shorelines (mainly through the formation of deltas) when sediment supply exceeded accommodation space created by basin subsidence. Just under 30 billion BOE has been discovered in progradational reservoirs; an estimated 4.1 billion BOE remains to be found.

The second environment in terms of discovered resources and first in undiscovered potential is the "fan" environment. These are facies that occur beyond the shelf edge during both high and low stands of sea level.

While relatively coarse grain sizes and poor sorting can preserve porosity and permeability, these reservoirs are notoriously heterogeneous in quality and in areal extent. Nevertheless, these deeper, underexplored facies represent the leading model for exploration in the gulf. While only 11.5 billion BOE has been discovered in fan environments, 18 billion BOE is expected to be found in the future in these types of reservoirs.

The MMS results support two technological generalizations about the direction of exploration in the Gulf of Mexico. First is that proposing fan environments as the leading location for new discoveries puts a high premium on both stratigraphic modeling of reservoir distribution, continuity and quality and on seismic techniques for identifying these elusive sand bodies. The good news is that, although these reservoirs tend to be deeper in the subsurface, a large part of the remaining potential is on the shelf in shallow water.

The second technological consequence of the MMS assessment is that the other large prize is in deep water. Here, the last 10 years have demonstrated hydrocarbon potential with a string of discoveries, most with high liquids contents. However, because of the cost of operating in these locations, minimum economic field sizes are substantially higher than they are on the shelf. Therefore, the technological premium is on platform and facilities cost reduction.

Conclusions

The analysis above, building on the results of the 1995-96 National Oil and Gas Assessment, supports several conclusions about the exploration potential of the U.S. when viewed with an eye toward the top plays in the country.

1. The plays with the largest endowments of conventional, technically recoverable undiscovered oil and gas are in Alaska, most of them in the arctic, both on and offshore. These plays also are expected to contain the largest undiscovered fields in the country. While their geologic potential is high, their exploration potential is qualified by economic and political constraints.

2. The three most promising classes of Lower 48 onshore and state waters plays are:

  • The oil and gas potential of the thrust environments of the northern Rocky Mountains;

  • Stratigraphic and depth extension of established gas plays in the Gulf Coast and Anadarko basins; and

  • Several highly prolific, mature plays where the tail-end of exploration is "fat" enough to support a large number of future discoveries and substantially larger than average-sized exploration targets.

3. About 12-20 plays are scattered around the Lower 48 that have relatively high exploration potential. Some of these are beyond the reach of exploration for policy and practical reasons, but most are not.

4. The Gulf of Mexico is the most important domain for offshore exploration in the U.S. The most prospective plays are in the Pleistocene. Some of these plays combine shallow water depth opportunities on the shelf, targeted heavily toward distal sedimentary environments that require deep drilling. They also include shallower target depths in deepwater environments.

5. Very attractive exploration plays are spread throughout the U.S. in a wide variety of geologic, logistic, and economic environments. While the very largest undiscovered fields are concentrated in the arctic and offshore, onshore Lower 48 provinces contain billions of barrels of oil and trillions of cubic feet of gas in their top plays.

End Part 2 of 3 Parts