Guest geological storytelling by David Middleton
You see the story yet? It’s all pretty much here.
In a language you can’t yet understand, but it’s here.
A tale of upheaval and battles won and lost.
Gothic tales of sweeping change, peaceful times, and then great trauma again.
And it all connects to our little friend.
That’s what we are, we geologists.
That’s what you gentlemen are going to become.
And how does this relate to the moon? From 240,000 miles away you have to give the most complete possible description of what you’re seeing.
Not just which rocks you plan to bring back but their context.
That and knowing which ones to pick up in the first place is what might separate you guys from those little robots.
You know, the ones some jaded souls think should have your job.
You see, you have to become our eyes and ears out there.
And for you to do that, you first have to learn the language of this little rock here.
–David Clennon as Dr. Leon (Lee) Silver, From the Earth to the Moon, Episode 10, Galileo Was Right, 1998
HBO’s 1998 From the Earth to the Moon miniseries was a sort of follow-on to the great movie Apollo 13… It’s a must see for space program fanatics. I particularly like this episode because my childhood interest in the space program led me toward the sciences and ultimately geology. Future Apollo 17 astronaut Harrison “Jack” Schmitt recruited his former field geology professor to train the Apollo 15 lunar module team and their backup crew how to become field geologists. It reminds me of why I love geology so much. I’ve also had the great honor of meeting Dr. Schmitt at the 2011 American Association of Petroleum Geologists convention in Houston. Shaking hands with someone who not only walked on the Moon, but also got to throw a rock hammer farther than any geologist ever has before or since, was pretty fracking cool.
While we usually aren’t 240,000 miles away from the stories we are trying to unravel. We are often 10,000 to 20,000 feet above the rocks that contain the story. In many cases, these rocks have no surface outcrops. Apart from well cuttings, sidewall cores and occasional whole cores, we never actually see the rocks. We often have to unravel the story from gravity, magnetic and seismic surveys, electric logs of wells that have been drilled into these rock formations, our imaginations and a firm understanding of “the language” of the rocks .
The Jurassic Period (Oxfordian) Norphlet formation is a great example of geological interpretation and storytelling…
In 1979, NASA’s SkyLab was still in orbit; Jimmy Carter was President; the US was suffering from the Second OPEC Oil Crisis; the Sony Walkman was introduced; and Terry Bradshaw’s Pittsburgh Steelers took Super Bowl XIII from the Dallas Cowboys. I was Exploration Manager for Mobil in New Orleans, and the Mobile Bay Field was discovered.
Today, Jimmy Carter has been a former President for over 30 years; the Walkman is an antique; and Terry Bradshaw is a 61‐year old actor and sports commentator. However, Mobile Bay is still producing gas and will continue to do so for at least another thirty years.
An oil or gas discovery often comes from a combination of luck, sound technical and management decisions, overcoming legal, environmental, and operational difficulties ‐ and perseverance at critical times. This story of the discovery of gas at 20,450 feet in the Mobile Bay #76‐1 well in 1979 includes all of the above. Located in the very heart of a major Gulf Coast recreational and historical area ‐ an offshore Civil War Battlefield no less ‐ Mobile Bay #76‐1 presented a unique set of technical and environmental challenges. This presentation describes the technical aspects of how this discovery came about, but unlike most such papers, the passage of time allows the author an opportunity to provide a new generation of geologists a glimpse of how the industry and the technical story has evolved. The real story here is one from a personal and human perspective and how the character and experience of some of the individuals involved had such a great impact on the successful and surprising outcome.Weldon G. Frost. Adapted from oral presentation at forum, History of Petroleum Geology, at AAPG Annual Convention and Exhibition, New Orleans, April 11, 2010
I was a senior in college when Mobil made the Mobile Bay discovery.
Before they could even spud Mobile Bay #76‐1, Mobil had to wage a legal battle with numerous groups opposed to drilling in Mobile Bay. Mobil won its legal battle under the conditions that the well would be totally zero-discharge, including collecting any rainwater that fell on the rig, and had to post a $55 million bond. Then they had to overcome Hurricane Frederic and the challenges of drilling a 20,000′ high pressure-high temperature (HPHT) well in 1979. However, it was worth taking on the challenges. The Mobile Bay #76‐1 encountered a 412′ gas column in the Jurassic Norphlet formation, with an initial test rate of 12.6 million cubic feet of natural gas per day (mmcf/d). It was the discovery well for the Mary Ann Field, 163 TCF (trillion cubic feet) of natural gas from 1979-1994, and the Mobile Bay Norphlet play, 693 TCF from 1979-1994.
Correction, the italicized passage should be:
It was the discovery well for the Mary Ann Field, 163 BCF (billion cubic feet) of natural gas from 1979-1994, and the Mobile Bay Norphlet play, 693 BCF from 1979-1994.
The exploration well success rate was remarkably high, particularly for a Jurassic play, especially since the discovery was “accidental”.
High Exploration Success Rate Seventeen Norphlet wells were spudded between 1981 and 1984, 13 of which became gas discoveries-an extraordinary accomplishment before 3D seismic. Besides Mobil’s Mary Ann discovery, three state Norphlet discoveries and one federal Norphlet discovery subsequently became development projects based on discoveries before year end 1984.
Through year end 1997, 75 Norphlet wells have been drilled in state and federal waters off Coastal Alabama-28 exploration, the balance delineation and development wells. These wells discovered 20 gas fields . Twenty Norphlet discoveries for 28 exploration wells represents a 71 percent success ratio . All but three of discovered fields are producing gas. Mobil abandoned the West Dauphin Island Field as uneconomic . Exxon maintains its 867 Field lease and plans to produce it in 1998 . Chevron and its partners, Conoco and Murphy, have filed a Development and Production Plan to develop Destin Dome. Exxon has drilled the most wells, 20; Mobil has drilled 19.
Mobil amended its original application to change the target from 16,500 to 21,500 feet-changing the target from Smackover to Norphlet-two years after the original application. One wonders what would have happened if Mobil had drilled to Smackover at 16,500 feet in 1970. Mobil’s decision to drill below the Smackover to the Norphlet in their original exploration of Lower Mobile Bay has been a bonanza for State of Alabama coffers and American consumers, even if the prices received for the gas have been sharply lower than operators anticipated . The Coastal Alabama/Panhandle Florida Norphlet trend is one of the most important U. S . gas producing regions.Wade et al, 1999
The play is still producing, but the most recent Mobile Bay discovery was in 1992 (Seni et al., 1997). It is now well on down the decline curve and the Mobile Bay 76 “A” platform was “reefed” in 2003.
Unfortunately for Chevron, Conoco and Murphy the State of Florida blocked the development of their Destin Dome discoveries.
While the Norphlet formation in Mobile Bay and Destin Dome was too hot for oil (>400 °F), recent deepwater Norphlet discoveries have encountered oil. In deepwater, a shallower geothermal gradient has kept the rocks cooler (~280 °F) since the Jurassic Period (Nadeau, 2015). Shell’s 700 million barrel Appomattox discovery (~7,000′ water depth) is expected to come on production in late 2019 or early 2020. Recent geochemical analyses indicate that the Mobile Bay’s Norphlet gas was actually oil as recently as 120 million years ago (Mankiewicz et al., 2009).
Weldon Frost’s presentation is available online at the AAPG’s Search and Discovery website. It’s a great story and it’s part of a larger story…
“A tale of upheaval and battles won and lost. Gothic tales of sweeping change, peaceful times, and then great trauma again.”
Well maybe not Gothic… but definitely “tales of sweeping change, peaceful times, and then great trauma again.”
The Norphlet formation was deposited about 150 to 155 million years ago in a salt-filled desert basin that would soon become the Gulf of Mexico. It was a massive complex of sand dunes and interdunal wadis stretching from Ark-La-Tex to M-A-Fla to the deepwater of the Gulf of Mexico. In the Ark-La-Tex (Southern Arkansas, East Texas and North Louisiana) the Norphlet was deposited as alluvium (sediments deposited by rivers, streams, lakes, etc.) In M-A-Fla (Mississippi, Alabama and Florida) and the Gulf of Mexico, it was deposited by eolian (wind-driven) processes. Norphlet production is from massive ancient sand dunes… at depths ranging from 20,000′ to 30,000′ feet below sea level.
This raises two questions:
- How did sand dunes, deposited in a desert, 150 million years ago, wind up buried under 20,000′ of rocks and sediments, under the ocean in water depths exceeding 7,000′?
- How can geologists possibly know these were sand dunes? There are no surface outcrops of the Norphlet formation.
I’ll try to answer the second question first.
How do geologists know that the Norphlet formation was deposited as sand dunes?
We know what modern sand dunes look like.
We know how they form.
We also have examples of ancient sand dunes that are exposed in outcrops. The Jurassic Navajo Sandstone in Zion National Park is a magnificent example of the Norphlet’s slightly older Western cousin.
We also have a pretty good idea of what to look for in sandstone to determine its nature of deposition. Steep cross-bedding is a common feature of sand dunes.
The Norphlet sand dunes were so quickly buried by Upper Jurassic Smackover formation, that the geomorphology (land forms) of the dunes have largely been preserved for over 150 million years.
By observing modern geological processes we can usually deduce the depositional environment (paleo-climate and paleo-geography) of sedimentary rocks.
How did the Norphlet sand dunes get buried under 20,000′ of rock under the ocean?
For starters, the Gulf of Mexico (GOM) did not exist 150 million years ago, at least not in anything remotely similar to its current configuration.
The Gulf of Mexico is a small ocean basin lying between the North American plate and the Yucatan block. Following initiation in the Middle Jurassic, sea-floor spreading continued approximately 25Myr. Spreading was asymmetric, creating a broad area of attenuated transitional continental crust beneath the northern basin. Initially, widespread, thick salt deposits accumulated across much of the basin; mobilization of this salt by subsequent sedimentary loading has created a complex suite of gravity tectonic structures. Most salt is now allochthonous, forming extensive stocks and canopies. By the end of the Mesozoic, thermal subsidence had created a deep basin floor, flanked by continental shelves. The resultant basin contains a succession of Late Jurassic through Holocene strata that is as much as 20km thick. Sediment supply from the North American continent has filled nearly one-half of the basin since its inception, primarily by offlap of the northern and northwestern margins.
During the Late Triassic Period, the North American and South American plates began to pull apart, creating a rift basin where the modern US Gulf Coast and Gulf of Mexico basins exist today (Smith et al., 2018). During the Middle Jurassic Period the basin was intermittently open to the Pacific Ocean and would fill with seawater. When the openings were closed, the proto-Gulf of Mexico would dry out, depositing thick layers of halite (rock salt) and other evaporites, predominately anhydrite (Dribus et al., 2008).
The Gulf of Mexico has always lain in the hot, arid subtropics. As a small ocean basin peripheral to the Atlantic, the Gulf has experienced extended geological intervals when connection to the world ocean was restricted. The first, and most dramatic result of aridity and restriction was deposition of widespread salt, the Louann, over much of the basin floor (Fig. 1B). This salt layer formed the foundation onto which subsequent sediment would be deposited. Salt deposition continued for several millions of years, creating a unit as much as several kilometers thick. Later, the arid tropical climate facilitated widespread deposition of limestone on shallow-water platforms whenever clastic sediment supply to the basin was low (Fig. 2, pdf). Later periods of restricted circulation in the Gulf favored burial and preservation of organic matter in marine sediments, setting the stage for formation of rich petroleum source rocks.
Galloway et al., 2009 Geo ExPro
The Jurassic Louann Salt formation underlies much of the Gulf Coast and Gulf of Mexico.
The Norphlet formation was deposited on top of the Louann Salt. Shortly after Norphlet deposition, a protracted period of sea level rise began. The Upper Jurassic Smackover formation is a carbonate sequence deposited during a marine transgression and high stand across much of the Gulf Coast and Gulf of Mexico. The Smackover exhibits a wide variety of facies ranging from organic-rich mudstones, to oolitic shoals, to patch reefs. It is one of the most prolific oil & gas reservoirs from East Texas to the Florida Panhandle. It is also possibly the most prolific source rock in the entire Gulf Coast/Gulf of Mexico Region (Walkinshaw, Norphlet). The Norphlet is generally only productive where the Smackover lacks porosity (Walkinshaw, Smackover). The Smackover is both the source and the seal for Norphlet oil & gas reservoirs.
Dr. William Galloway, of the University of Texas Jackson School of Geosciences, summarized the depositional history of the Gulf Coast/Gulf or Mexico in this must read paper (I must read it, because he let me have a copy of it)…
Depositional history can be generalized in seven phases: (1) Middle-Late Jurassic evaporite and carbonate deposition in a broad, shallow, restricted to open marine basin. (2) Latest Jurassic-Early Cretaceous sand-rich clastic progradation from the northern margins. (3) Late-Early Cretaceous development of a rimmed carbonate shelf. (4) Late Cretaceous mixed clastic and carbonate aggradation of the continental margins. (5) Resurgent Paleogene clastic progradation and filling centered in the NW basin. (6) Miocene progradation and basin filling centered in the central and NE Gulf. (7) Late Neogene climatically and eustatically influenced progradation along the central Gulf margin. In contrast to the broad, progradational sediment wedge of the northern Gulf, the Florida margin is a primarily aggradational carbonate platform.
Figure 7 clearly demonstrates the importance of sea level cycles in the depositional history of the GOM. Also note that all of the source rock formations were deposited when atmospheric CO2 was above 1,000 ppm and the Earth was considerably warmer than it is today. The temperature and CO2 plots have 10 million year resolutions; they are highly smoothed.
Sediment loading resulted in a series of down-to-the-basin growth faults, extensive deformation of the Louann Salt and eventual mobilization of the salt into “allocthonous” bodies, which caused compressional features to form where the salt was primarily moving laterally.
This is a generalized Upper Cretaceous paleogeography map of the Gulf of Mexico with an isopach (thickness contours) of the total Cenozoic sedimentary section (Seni et al., 1997).
Even though the high temperature Norphlet gas pays of Mobile Bay and Destin dome are at about the same depth below the seafloor as the lower temperature Norphlet oil pays in Mississippi Canyon and DeSoto Canyon, the overburden is considerably different.
100 million years of generally rising sea level, followed by 50 million years of generally falling sea level… All punctuated by high frequency cycles of rising and falling sea level… Accompanied by down-warping and stretching of thin transitional continental crust and halokinesis (salt movement) buried a 150-million year old desert 20,000′ below the seafloor of the Gulf of Mexico… Unless sea level rise wasn’t related to climate change prior to 1988…
Climate change buried a desert 20,000 feet beneath the Gulf of Mexico seafloor!
How about that, geology fans?Brett Cullen as Apollo 15 Mission Commander Dave Scott, From the Earth to the Moon, Episode 10, Galileo Was Right, 1998
“A tale of upheaval and battles won and lost.”
Mobil’s battles to drill Mobile Bay #76-1 didn’t end the “war.” The first five deepwater Norphlet wells were either dry holes, filled with solid hydrocarbons or contained insufficient volumes of oil. Even when Shell made their 700 million barrel Appomattox discovery, its development was not a sure thing. During the price crash of 2014-2016, Shell’s Appomattox team was able to innovate, reduce costs and improve efficiencies to the point that the project’s break-even price was less than $50/bbl.
Other battles remain to be fought. Most of the Norphlet play is off limits to exploration and production (E&P) activities because it is in the Eastern Gulf of Mexico (AKA Offshore Florida); as is almost all of the Mesozoic resource potential in the GOM OCS… So, the battle goes on…
I’ll just close with another space theme and one of my favorite sources for pertinent quotes, Tim Allen… “Never give up! Never surrender!”
Expanded References (the post would have been too long if I covered all of them)
“Areas Under Restriction.” Bureau of Ocean Energy Management, www.boem.gov/Areas-Under-Moratoria/.
Berner, R.A. and Z. Kothavala, 2001. GEOCARB III: A Revised Model of Atmospheric CO2 over Phanerozoic Time, American Journal of Science, v.301, pp.182-204, February 2001.
Chowdhury, Abu. “Salt Geology and Hydrocarbon Plays in the Northeastern Gulf of Mexico.” Search and Discovery Article #10215, AAPG Datapages, Inc., 10 Nov. 2009, www.searchanddiscovery.com/documents/2009/10215chowdhury/index.htm.
Douglas, Scott W. “The Jurassic Norphlet Formation of the Deep-Water Eastern Gulf of Mexico: A Sedimentologic Investigation of Aeolian Facies, Their Reservoir Characteristics, and Their Depositional History.” Baylor University, 2010. (A great master’s thesis… makes me want to go back to college… Except they don’t have keg parties now… So maybe I won’t.)
Dribus, John R., et al. “The Prize Beneath the Salt.” Oilfield Review, Schlumberger, 2008.
Francis, Thomas. “Appomattox: an Energy Project That Defied the Odds.” Shell Global, www.shell.com/inside-energy/an-energy-giant-born-against-the-odds.html.
Frost, Weldon G. “The Somewhat Accidental Discovery of the Mobile Bay Gas Field: A Story of Perseverance and Good Fortune .” Search and Discovery Article #110133 , AAPG atapages, Inc., 16 June 2010, www.searchanddiscovery.com/documents/2010/110133frost/ndx_frost.pdf?q=%2BtextStrip%3Anorphlet+-isMeetingAbstract%3Amtgabsyes.
Galloway, William. (2008). “Chapter 15 Depositional Evolution of the Gulf of Mexico Sedimentary Basin”. Volume 5: Ed. Andrew D. Miall, The Sedimentary Basins of the United States and Canada., ISBN: 978-0-444-50425-8, Elsevier B.V., pp. 505-549. (Special thanks to Dr. Gallloway for sending me a copy of this)
Galloway, William E., et al. “Gulf of Mexico.” GEO ExPro, 2009, www.geoexpro.com/articles/2009/03/gulf-of-mexico.
Hood, Kenneth C., L. M. Wenger, O. P. Gross, S. C. Harrison, 2002. “Hydrocarbon Systems Analysis of the Northern Gulf of Mexico: Delineation of Hydrocarbon Migration Pathways Using Seeps and Seismic Imaging”, Surface Exploration Case Histories: Applications of Geoschemistry, Magnetics, and Remote Sensing, Dietmar Schumacher, Leonard A. LeSchack
Karlo, John F, and Robert C Shoup. “Classifications of Syndepositional Systems and Tectonic Provinces of the Northern Gulf of Mexico.” Search and Discovery Article #30004, AAPG Datapages, Inc., 2000, www.searchanddiscovery.com/documents/karlo/index.htm.
Mancini, Ernest A. & Bearden, Bennett & M. Mink, Robert & P. Wilkerson, Richard. “Norphlet Formation (Upper Jurassic) of Southwestern and Offshore Alabama: Environments of Deposition and Petroleum Geology.” AAPG Bulletin, vol. 69, 1985, doi:10.1306/ad462b14-16f7-11d7-8645000102c1865d.
Mankiewicz, Paul J., et al. “Gas Geochemistry of the Mobile Bay Jurassic Norphlet Formation: Thermal Controls and Implications for Reservoir Connectivity.” AAPG Bulletin, American Association of Petroleum Geologists, Oct. 2009, archives.datapages.com/data/bulletns/2009/10oct/BLTN08171/BLTN08171.HTM.
Mello, Ulisses & D. Karner, Garry & Anderson, Roger. (1995). Role of salt in restraining the maturation of subsalt source rocks. Marine and Petroleum Geology. 12. 697-716. 10.1016/0264-8172(95)93596-V.
Miller, Kenneth & Kominz, Michelle & V Browning, James & Wright, James & Mountain, Gregory & E Katz, Miriam & J Sugarman, Peter & Cramer, Benjamin & Christie-Blick, Nicholas & Pekar, S. (2005). “The Phanerozoic Record of Global Sea-Level Change”. Science (New York, N.Y.). 310. 1293-8. 10.1126/science.1116412.
Nadeau, P H. (2015). Deepwater Horizon Study Group: Lessons learned from the… Deepwater Horizon Study Group 3 Working Paper – January 2011 10.13140/RG.2.1.1447.3125.
Royer, D. L., R. A. Berner, I. P. Montanez, N. J. Tabor and D. J. Beerling. CO2 as a primary driver of Phanerozoic climate. GSA Today, Vol. 14, No. 3. (2004), pp. 4-10
Schoell, Martin & Carlson, Robert. (1999). Diamondoids and oil are not forever. Nature. 399. 15-16. 10.1038/19847. (Nor are diamondoids diamonds. They are hydrocarbons… black gold, Texas tea)
Seni, S. J., and others, editors, 1997, Atlas of Northern Gulf of Mexico Gas and Oil Reservoirs, Volume 1 – “Miocene and Older Reservoirs”: The University of Texas at Austin, Bureau of Economic Geology, Atlas of Major Oil and Gas Reservoirs, 199 p. (Affectionately referred to as “The Old Testament” around here. Volume 2 – Pliocene and Pleistocene Reservoirs is “The New Testament”)
Smith, Thomas, et al. “New Insights Revealed Into the Hydrocarbon Potential Along the Gulf of Mexico Conjugate Margins.” GEO ExPro, 2018, www.geoexpro.com/articles/2018/12/new-insights-revealed-into-the-hydrocarbon-potential-along-the-gulf-of-mexico-conjugate-margins.
United States, Department of the Interior, Bureau of Ocean Energy Management. “ Oil and Gas Resources as of Jan 1, 2014: OCS Report BOEM 2017-005.” Oil and Gas Resources as of Jan 1, 2014: OCS Report BOEM 2017-005, BOEM, 2017. https://www.boem.gov/BOEM-2017-005/
Wade, W.W., J.R. Plater, and J.Q. Kelley . 1999. History of Coastal Alabama Natural Gas Exploration and Development, Final Report. OCS Study MMS 99-0031 . U.S. Dept. of the Interior, Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, La. 189 pp.
Walkinshaw, Steve. “ THE NORPHLET FORMATION ARKOSES, DUNES AND AN IMPORTANT RULE OF THUMB.” Vision Exploration, L.L.C. – The Norphlet Formation, www.visionexploration.com/norphlet.htm.
Walkinshaw, Steve. “THE SMACKOVER OF ALABAMA & MISSISSIPPI A PROLIFIC SOURCE ROCK AND RESERVOIR.” Vision Exploration, L.L.C. – The Smackover Formation, http://www.visionexploration.com/smackover.htm.