OCTOBER 2015

The “Better Business” Publication Serving the Exploration / Drilling / Production Industry Basement Help Locate Pay

By Corine Prieto producing and projected sites can be northern Gulf of Mexico over the past identified. two decades. HOUSTON–Establishing basement Today’s cost for acquiring seismic The study confirms that a financial tectonics should be regarded as the first data over an optimum areal extent is $15 risk reduction factor can be applied when step in evaluating an international fron- million. Completion costs for a deep- a correlation between the position of an tier. target drilling operation average $1 million oil and gas prospect and the magnetical- More than two decades of data correlate per 1,000 feet, or generally a minimum ly-based basement structure map is made. the relationship between oil and gas fields of $30 million. New Wilcox and Norphlet Many operators have found the result and discovery wells to regional structural discoveries are at depths near 30,000 has led to better decisions about how basement characteristics in the northern feet. At these prices, cost makes a strong best to use their drilling and development Gulf of Mexico, and demonstrate the ra- case for including all information, in- budgets. tionale for utilizing regional basement cluding the basement structure, in the structure as a tool for evaluating explo- analysis and economic assessment of The Theory ration prospects. prospective targets. Despite the technological explosion Prospectivity continues to expand in Based on the premise that a financial of recent years, the most cost effective the northern Gulf of Mexico as drilling risk reduction factor can be applied when concept defining the location of oil and technology reaches greater depths. How- a correlation is made between the position gas fields arguably is still the “anticline ever, the costs and risks associated with of an oil and gas prospect and the mag- theory,” i.e., closed structural high at deep- and ultradeepwater exploration and netically-based basement structure map, reservoir level. Such a structural anomaly drilling are increasing, and it has become a research study was conducted to correlate must be located in the pathway of oil and apparent that using regional basement the relationship between oil and gas fields gas, which migrates both laterally and structure as an evaluation tool can help and discovery wells to regional structural vertically, in order to maximize its trapping reduce both cost and risk. The position basement characteristics, as well as to potential. of oil and gas fields can be a vital param- demonstrate the rationale for utilizing re- On the regional scale, a structural high eter in predicting the viability of explo- gional basement structure as an evaluation must be in close proximity to a structural ration sites, when the correlation between tool. The study used data amassed in the low (, depocenter, or sub-basin). From a basinwide perspective, high struc- FIGURE 1 tural anomalies become the focal points Jack & St. Malo Discovery Locations of migrating oil and gas. Depth Range is 35,000 (Red) to 50,000 (Blue) Feet Regional (low) structural anomalies are the depocenters of generating “kitchens.” In most cases, basement anom- alies underlie and cause regional basinwide sedimentary structural anomalies. This is the primary rationale underlying the practical application of basement maps as exploration tools. Less accepted–or merely less known– is that in any producing basin, oil and gas are concentrated in specific areas. In fact, 75 percent or more of the oil and gas in a producing basin is contained in only 25 percent or less of the total basin

Reproduced for Integrated Geophysics Corp. with permission from The American Oil & Gas Reporter www.aogr.com Special Report: Gulf of Mexico Activity

• 78 discoveries, or 30 percent, are situated in basement lows, such as Zinc, Holstein, Power Play and Tahoe. Out of the 258 northern Gulf of Mexico field discoveries cataloged, the results to date show that approximately 70 percent of the oil and gas reserves are located either on steep/faulted flanks or on top of basement structural highs. This leads to the observation that structural lows contain thicker sediment columns, and therefore thicker sections of potential oil and gas source beds. It is established that oil and gas gen - erated in regional lows migrate updip onto regional highs. Regional structural highs adjacent to a deep basement struc - Northern Gulf of Mexico deepwater discoveries correlated to basement structure are posted on a bathymetry map. tural low become attractive sites for oil accumulation. Structural highs located between two adjacent basement lows area. two decades of diligent analysis of depth offer special attraction for migration path - Further, structural positive (high) anom - estimates derived from various vintages ways from both sides. Regional low areas alies near structural negative (low) anom - of aeromagnetic surveys. This offshore become the sites for increased sediment alies are the preferred targets. Typical basement map has been correlated to deposition with a preferred content of examples are the Jack (Walker Ridge published , and has been calibrated oil/gas source beds (depocenters). Block 759) and St. Malo (Walker Ridge to available refraction data, onshore struc - Regional highs become the sites for Block 678) discoveries, which were drilled tural ties, and well control. Published reservoir development, especially on the in ±7,000 feet of water and through salt data identify the location of more than flanks where sand units pinch out against bodies greater than 8,000 feet thick. Sig - 258 Outer Continental Shelf blocks as - existing topographic and structural highs. nificantly, each discovery can be correlated sociated with oil and gas discoveries. Finally, oil and gas that is generated ther - to a basement high block on the depth- The study first plotted the acreage mally in regional structural lows migrates to-basement structure map (Figure 1). blocks carrying the fields reported by the updip to regional structural highs. The basement depth at the Jack well lo - Bureau of Ocean Energy Management, cation is 45,000 feet; the depth at the St. Regulation and Enforcement, as well as Logical Connection Malo structure is 38,000 feet. the reported discoveries. The location For many skeptics, the absence of a and the geological-structural framework logical connection, or rather the lack of Methodology of these productive blocks were correlated data concerning a connection between The study’s research objective was to to a magnetically-based basement struc - basement structure and the required struc - correlate the relationship between oil and tural interpretation and classified into tural situation at the level of deeper Juras - gas fields and discovery wells to regional three primary categories: sic, Cretaceous and Eocene source beds, structural basement characteristics, and • OCS blocks on the top of basement is critical. Such a connection is required to demonstrate the rationale for utilizing structural highs; to assign a higher exploration value to regional basement structure as an evalu - • OCS blocks in steep and faulted the basement structure map. ation tool. flanks of basement structural highs; and Unfortunately, there are no maps gen - Simply stated, the position of known • OCS blocks in basement structural oil and gas fields can be a vital parameter lows. erated with confidence from these deeper in predicting the viability of exploration Furthermore, in terms of productive levels today. Therefore, analyzing prospec - sites when the correlation between pro - acreage blocks analyzed, of the 258 tive acreage blocks must be restricted to ducing and projected sites can be identified. acreage blocks in the Gulf deepwater re - the relationships observed between the In the deepwater regime of the northern gion publicly reported as having discovery magnetically-based basement structure Gulf of Mexico, the availability of a reli - wells or as being part of an oil/gas field location and distribution of productive able regional basement structure map and development operation, the study’s findings acreage blocks, and to discovered oil/gas a documented number of oil and gas dis - determined the following relationships reserves. coveries provides an opportunity to test to basement structure: Basement structures in the northern the strategy. • 116 discoveries, or 45 percent of Gulf are the result of multiple deep- Two types of databases are utilized total samples, are located on steep and seated tensional and faulting at the for the statistical analysis correlation: faulted flanks of basement high blocks, basement level. Subsequent basement • A magnetically-based basement such as the Jack, St. Malo and Appomattox motions, as well as the impact of topo - structure (depth and contours) map; and discoveries. graphical differences at the level of sedi - • A tabulation of oil and gas field • 64 discoveries, or 25 percent, are mentation during Mesozoic and Lower discoveries from several published sources. located on basement structure highs, such Tertiary times, originally led to the de - The basement map is the product of as Vicksburg, Troika and Dawson. velopment of regional lows and regional Special Report: Gulf of Mexico Activity

FIGURE 2 had mapped the Gulf’s basement as planar, sloping down to the abyss. The significant Appomattox & Vicksburg Discovery Locations seismic assurance to the structural con - Depth Range is 40,000 (Red) to 60,000 (Blue) Feet figuration did not develop until the arrival of the 14-second seismic section. It was then that serious interest devel - oped in the structural framework of the Gulf of Mexico. New, high-resolution aeromagnetic data were acquired from 1992 to 2009, such that today, the northern Gulf is covered with 0.5- to 0.25-mile by 1.0-mile surveys. The correlation between magnetical - ly-based basement structure and oil/gas production has been demonstrated in many basins around the world. A few ex - amples are Argentina, the Austrian Vienna Basin, China (onshore/offshore), Ecuador, the Hungarian Plains, India (western/east - ern offshore), Peru, Sumatra (northwest - ern), former Soviet Union, and Yugoslavia. The statistics we started to gather in 2000 indicate the Gulf of Mexico can be added to the list. The 2015 analysis indi - cates that approximately 70 percent of the Gulf’s oil- and gas-field discovery reserves can be related to structure de - scribed as either steep/faulted flanks or highs. The quality of basement structure data. The basement depth at the Vicksburg drape-over of basement highs. Early dis - mapping has improved continuously with well location is 48,000 feet. The depth at coveries in the deepwater Gulf of Mexi - higher quality data, an abundance of data, the Appomattox structure on the down- co–Tahoe (Viosca Knoll Block 783 in and more refined interpretation tech - thrown side of the is 54,000 feet. 1984), Bullwinkle (Green Canyon Block niques. 65 in 1983), Auger in 1987, and Mars in Sequence Of Events Deepwater Gulf of Mexico discoveries 1989–led to an awareness of its prospec - have verified that the basement structure IGC delivered the first version of a tivity by a vast majority of pioneer com - has influenced oil and gas migration. In Gulfwide basement structure in 1983. panies (Figure 3). July 2013, Shell Oil Co. announced that The structural configuration was based These initial discoveries flanked our its deepwater Mississippi Canyon Block on four- by eight-mile coverage of aero - interpretation of a ridge complex of struc - 393 Vicksburg discovery encountered magnetic surveys. tural highs with an average depth of more than 500 feet of pay at a depth of Since we had no basement control 44,000 feet and an average water depth 26,385 feet (water depth was at 7,446 within the Gulf, the data coverage provided of 5,200 feet paralleling the Louisiana feet). The Vicksburg discovery was a fol - an opportunity to tie to onshore structures. Coastline, which I called the “first buttress” low-up to the Shell/Nexen Appomattox We interpreted that the Louisiana and into the deepwater region. There are more discovery (February 2012), which en - Texas troughs reached depths of 65,000 examples, such as ExxonMobil’s Hadrian countered approximately 150 feet of oil feet, while the average depth across the discoveries in Keathley Canyon. The pay at a total depth of 25,851 feet. Gulf was close to 40,000 feet below sea Davy Jones discovery on the Louisiana The press release said a contingent level. At that time, the seismic community Shelf lies on what I interpret as a basement recoverable resource of 215 million barrels of oil equivalent was identified in the Jurassic Norphlet formation in the north - FIGURE 3 east of the Appomattox struc - “First Buttress” Northern Gulf Deepwater Discoveries ture. The well is located in Mississippi Depth Range is 35,000 (Red) to 60,000 (Blue) Feet Canyon Block 348 in 7,257 feet of wa - ter. The complexity of the basement struc - ture in the immediate area surrounding the wells is shown in Figure 2. This author has not been privy to the seismic interpretation, and can describe the base - ment environment only from an interpre - tation based on a high-resolution aero - magnetic survey and refraction velocity Special Report: Gulf of Mexico Activity

embayment, as do the Lower Tertiary forms). discoveries in Alaminos Canyon. Structural framework controls alter - This started our collection of discovery nating extensional and compressional dy - statistics published in various industry namics of the Louann Salt. The rate of sources. The 15-year collection is evidence sedimentation and the rate of subsidence that the Gulf of Mexico’s structural frame - are controlled by the structural configu - work is a critical and integral part of the ration and tectonics of the basement; i.e., hydrocarbon evaluation process. We are sedimentary regression and transgression aware that the oil and gas play within the through channels and fan systems. Un - sedimentary section differs with each dis - derstanding the structural fabrics of the CORINE covery, yet 70 percent correlate to base - basement and overlying sedimentary sec - PRIETO ment structure. tions can unravel areas of identical struc - Corine Prieto is founder and pres - Basement structure is defined as the tural developments in less well-explored ident of Integrated Geophysics Corp. topography and structural fabric (shear sectors of the Gulf of Mexico. Ì in Houston. She has 40 years of inte - zones and fault zones) within the crys - grated geophysical and geological ex - talline basement. These deepwater and Editor’s Note: Many have contributed perience in the Gulf of Mexico, onshore Shelf structures in the Gulf of Mexico to the IGC basement structural interpre - North American basins, the North Sea, are the result of uplifts, platforms, or tation and findings discussed in this Africa, the Middle East, the Far East, multiple deep-seated tensional and shear article, notably Michael Alexander, as - and the West Indies. Before founding faulting. sociate geophysicist; J.C. Pratsch, con - IGC in 1981, Prieto served five years The process of delineating regional sulting geologist; and Karim Aimadeddine, as exploration supervisor at Superior uplifted blocks (horsts) and depocenters geophysicist. CGG (formally Fugro Air - Oil Company and eight years as a associated with faults/ zones, en - borne of Canada) provided the aeromag - geophysicist at Mobil Oil. She holds ables companies to locate hydrocarbon netic database utilized in this undertaking. a B.S. in physics and math from the prospects (migration pathways). The Worldwide areas and basins discussed University of Texas at El Paso, and Wilcox and Norphlet plays appear to cor - are from publications that described the an M.S. in applied physics from the relate to basement embayments (plat - concentration of oil and gas. University of Toronto.