22748schD4R1.qxp:22748schD4R1 11/26/08 3:12 PM Page 4 The Prize Beneath the Salt Exploration and production activities in the deep and ultradeep waters of the northern Gulf of Mexico have led the way to the latest subsalt play. Lessons learned from this play may, in turn, open the way for subsalt exploration in other basins around the world. John R. Dribus New Orleans, Louisiana, USA Martin P.A. Jackson LOUISIANA Bureau of Economic Geology The University of Texas at Austin Lafayette Austin, Texas, USA Houston Jerry Kapoor Martiris F. Smith TEXAS Houston, Texas Galveston For help in preparation of this article, thanks to Joelle Fay and Mark Riding, Gatwick, England; and Chris Garcia, Mexico City, Mexico. TEXAS-LOUISIANA SHELF Q-Marine is a mark of Schlumberger. FLOWER GARDEN BANKS 100 500 100 1000 500 CENTRAL NORTHWEST1500 SLOPE SLOPE 2000 USA RIO GRANDE SHELF FAN 3000 ALAMINOS SI WALKER GS RIDGE CANYON BE 2000 T E N KEATHLEY ESC MEXICO E PERDIDO ARP T CANYON MEN M 3500 P CANYON R EAST A C 500 S MEXICO 1000 E SHELF O 1500 D I D 3000 R E 2500 P MEXICO BASIN 1000 4 Oilfield Review 22748schD4R1.qxp:22748schD4R1 1/8/09 2:13 PM Page 5 Successes in deep- and ultradeepwater prospects highly competitive arena, where choice blocks of to notable subsalt discoveries on the Shelf, such are spurring a resurgence in exploration in the offshore real estate can lease for more than as Mahogany; or in deeper waters, such as Gulf of Mexico. Announcements of significant US $100 million for a 10-year lease. But secrecy Gemini, Atlantis, Tahiti, Mad Dog and Pony; or discoveries or record-setting achievements have and competition are not the only impediments to in ultradeep waters, such as Thunder Horse, piqued interest in the activities of deepwater exploration in this area. St. Malo, Jack and Kaskida.2 operators, as evidenced by bidding competition Perhaps the greatest challenge is posed by Salt is a particular challenge for drillers, who at recent offshore lease sales. Exploratory thick layers of salt in the subsurface. Several must contend with high-pressure sediment drilling has also confirmed the presence of deepwater prospects lie beneath sheets of salt— 1 1. Close F, McCavitt RD and Smith B: “Deepwater Gulf of reservoir-quality sands that are much farther some up to 20,000 ft [6,100 m] thick. Like an Mexico Development Challenges Overview,” paper SPE from shore than expected. The discovery of undulating and tattered canopy, coalesced salt 113011, presented at the SPE North Africa Technical Conference and Exhibition, Marrakech, Morocco, hydrocarbons in several of these sands is fueling sheets extend from the US Continental Shelf to March 12–14, 2008. speculation about the tantalizing potential that the deepwater Continental Slope off the coasts of 2. The concept of deep water has evolved considerably might lie beneath deep Gulf waters. Texas and Louisiana. A similar subsalt province throughout the years. The US Department of Interior Minerals Management Service (MMS) originally defined Speculation is in large supply. Offshore lies well to the south, in less-explored waters off deep water as 200 m [656 ft]. This mark was later eclipsed operators diligently safeguard their expensive Mexico’s Yucatan Peninsula. by industry drilling trends, and now the deepwater stan- dard is set at 1,000 ft [305 m]. The MMS has designated and hard-won secrets of the deep and release In their pursuit of new exploration targets, depths greater than 5,000 ft [1,524 m] as ultradeep water. only the data required by governmental mandate. many offshore operators have had to drill In the Gulf of Mexico, the deepest waters are found in the Sigsbee Deep, whose estimated depths range from Press releases are carefully constructed to avoid through hundreds or even thousands of feet of 12,303 ft [3,750 m] to 14,383 ft [4,384 m]. revealing the full extent of discoveries in this salt to discover pay sands. Their efforts have led ALABAMA MISSISSIPPI Mobile Pensacola Gulfport Tallahassee New Orleans MISSISIPPI- ALABAMA SHELF100 500 N VIOSCA KNOLL Y O N A C O FLORIDA T W O S E SHIP SHOAL E S D T F M L 1000 O I S R S 1500 I ID WEST S A 100 S A I T S EWING BANK P W L P O FLORIDA Tampa I A 500 T P C E R W E A ES 2500V 3000 T N A SHELF L L FL Y E O 1000 Y R St. Petersburg O I MISSISSIPPI N D SLOPE A 2000 E GREEN L S U C CANYON N A T D N R E V A MISSISSIPPI P M L M P 2500 L E Y 100 R FAN EN A T C 2000 S Fort Myers E E E B 3000 S G SI km FLORIDA 0 100 200 300 0 100 200 PLAIN mi PECHE ESCAR Bathymetry lines represent 100m [330 ft] increments. AM PM 3500 C EN Map adapted from Taylor LA, Holcombe TL and Bryant WR: Bathymetry of the T Northern Gulf of Mexico and the Atlantic Ocean East of Florida. Boulder, Colorado, USA: National Geophysical Data Center, 2000. This US government publication is in the public domain: http://www.ngdc.noaa.gov/mgg (accessed October 23, 2008). 2000 1500 500 10 50 00 Autumn 2008 5 22748schD4R1.qxp:22748schD4R1 11/26/08 3:13 PM Page 6 system in the Gulf of Mexico. We also discuss how sands that were originally deposited on top of 60° salt have come to lie beneath it. Although this article focuses primarily on the intensively explored US waters of the northern Gulf of EURASIA Mexico, some of the principles described are also 30° relevant to other basins around the world. NORTH AMERICA Evolution of the Gulf of Mexico Basin Discoveries in the Gulf of Mexico have 0° challenged previous thinking regarding the occurrence of hydrocarbon-bearing sands that lie beneath great thicknesses of salt. This salt is AFRICA actually older than the sands that lie beneath it. SOUTH However, the salt has moved and in some cases 30° AMERICA created seals capable of trapping oil and gas. INDIA Understanding the geological complexities of AUSTRALIA these discoveries requires a step back in time. The evolving subsalt play in the Gulf of Mexico 60° ANTARCTICA is inextricably tied to the geologic history of the Gulf itself. This history goes back eons, before the > Earth during the Triassic period. The eventual breakup of the Pangean Gulf of Mexico existed, to a time after most of the supercontinent was manifested in part by rifting between the continental world’s continental plates had converged into a plates of Africa and the Americas, which eventually led to the formation of supercontinent known as Pangea (left). The the Gulf of Mexico. tectonic activity that followed would mold the Gulf basin and influence the distribution of sediments that subsequently filled it. inclusions or rubble zones as they drill through the Continental Shelf of the northern Gulf had In addition to unremittingly gradual tectonic salt bodies (see “Meeting the Subsalt Challenge,” been estimated at 1.2 billion bbl [190 million m3] and depositional processes in this basin, other page 32). It also poses substantial difficulties for of oil and 15 Tcf [435 billion m3] of gas.4 These forces were at work. The early history of the Gulf geophysicists as they attempt to image deep estimates do not include additional reserves that of Mexico was at times abruptly punctuated by structures beneath irregularly shaped salt have since been discovered in the deepwater catastrophic events that not only influenced the bodies. In salt, seismic waves can reach Miocene play—including Mad Dog, Pony, Tahiti formation of the Gulf, but also changed the world. velocities of 14,500 to 15,100 ft/s [4,400 to 4,600 and others—nor do they include ultradeepwater These events were remarkable in their scale, but m/s]—in some cases roughly twice the velocity subsalt discoveries such as Jack, Kaskida, were certainly not unique; one such cataclysm they would travel in surrounding sediments. This St. Malo and Thunder Horse. occurred just before the Gulf began to open. velocity contrast causes geophysical imaging This article describes the evolving subsalt Approximately 250 million years ago, much of problems that can mask underlying structures play in the northern Gulf of Mexico. We briefly life on Earth was extinguished. On land, insect and prevent geoscientists from determining the review the geologic processes that led to biodiversity plummeted, while plant and animal location or extent of potential reservoirs.3 deposition of the Louann Salt and the transport losses were even more severe, as 70% of land However, advances in seismic acquisition and of reservoir-quality sands into deeper reaches of species vanished. In the seas, trilobites, tabulate processing techniques are helping geophysicists the basin. We examine the role of salt and rugose corals, and nearly all crinoids became resolve problems that previously prevented mobilization and its effect on overlying sediment extinct, along with roughly 90% of all marine imaging beneath the salt (see “Shooting Seismic in the formation of traps and migration pathways species. Atmospheric oxygen levels dropped from Surveys in Circles,” page 18). necessary to complete an effective petroleum 30% to less than 15%. The process of drilling and producing a subsalt well requires significant planning, 3. Farmer P, Miller D, Pieprzak A, Rutledge J and Woods R: oxidizing conditions. The red color results from specks of “Exploring the Subsalt,” Oilfield Review 8, no. 1 iron oxide minerals in the sediments.