Gulf of Mexico
The Gulf of Mexico (GoM) FIGURE 1 is a long established prolific hydrocarbon basin located along the coast of the southern parts of the United States of America. This area is notable not only for energy resources located offshore but also it's supporting infrastructure and refining capability onshore.
GoM origin is proposed to be related to the extensive regional subsidence of more than 10,000 ft during the Cretaceous time and its isolation is due to the continuous coeval carbonate growth of Florida and Yucatan platforms. Large platform growth rates, compared to small depositions lead to the deepening of the GoM. Present-day GoM size is the result of a huge mass of Cenozoic deposits which many refer to as the Gulf Coast Geosyncline. However, the last major volume of clastics was deposited on the Mississippi cone in the early Holocene time. Recent deposits from the Mississippi and other rivers were laid down on the continental shelves and reaches the byssal gulf by turbidity currents.  Imaging the subsurface of the GoM has caused difficulty in the past years due to the complexity of structure and the large presence of salt domes. Mass accumulations of salt have hindered imaging with seismic and cause problems such as dim amplitudes, difficulties creating a velocity model and with these poor velocity models it causes the images near the salt body to be distorted. However, recent research in seismic techniques such as long source/ receiver offsets and prestack depth migration algorithms have made processing data near salt more accurate and reliable.
One of the more obvious challenges of developing prospects in the Gulf of Mexico is unpredictability of the weather. The Gulf is known to experience harsh storms and dangerous seasonal hurricanes which pose danger to ongoing operations and risks damaging existing equipment. This combined with the distance of existing and future wells also creates a problem with getting supplies and equipment to the location.
Many of the prospects in the GoM are located in a sub-salt environment. These salt accumulations make it impossible to obtain deep seismic resolution. Lack of seismic data makes it more difficult to plan and appraise prospects. The complex nature of the formations coupled with the great depth that must be drilled poses a multitude of problems for drilling design. Near the surface there are faults and unpredictable narrow veins of high pressure gas. Deeper into the basin lie large concentrations of salt anywhere from 7,000 to 20,000 feet thick. Theses salt canopies can be mobile/flowable and can contain random amount of trapped sediments. At the base of the salt rapid pressure changes can occur. Additionally, "thief zones" can be encountered which can cause massive fluid loss. At the desired reservoir level high temperatures and pressures are experienced coupled with a low natural flow ability.
The next set of issues deals with completion of wells. As previously mentioned reservoirs tend to exhibit low natural flow ability. These reservoirs need to be fractured and packed with proppant to be able to produce acceptable levels of flow. These issues are further exacerbated by the HPHT (high pressure high temperature) environment.
Pipeline systems are also affected by the cold conditions of the seabed. Relatively low ocean temperatures can cause multiphase slug flow which and also the buildup of wax. These conditions can cause flow assurance issues and structural integrity of pipelines can be damaged by slugging and corrosion.
The GOM is a mud-dominated basin that also has impermeable layers of salt which acts as efficient seal. Salt has negligible permeability which makes it act as the perfect seal. For example the Tahiti field's primary reservoir is of Miocene age and it is overlain by a thick and highly complex salt canopy. The field was created by a three-way structural trap that seals against the salt canopy. The Green Canyon area of the GoM located along the Northern GoM continental slope, the Sigsbee Escarpment provides the seal for trapped hydrocarbon. The Sigsbee Escarpment is made of highly consolidated low permeability sediment and was formed by the upward movement of salt. Salt however can compromise seal ability through its own upward movement and it penetrating and shifting shale sediments acting as a seal as seen in the Auger basin of the GoM.
Hydrocarbon Source Rock
The Gulf of Mexico basin contains four distinct thick source rock intervals pictured to the right in the figure 2. These source rocks are composed of Algal mats that grew around the edges of the GoM, however, over time the massive deposition of sediments from the Rocky Mountains rising out of the ground buried these algal. As the continued to get buried these algal cooked from the blistering temperatures of the earth and got turned into a source rock. Turonian source rock is dominantly composed of marine organic matter that produces low-sulfur oils. Direct oil-to-source-rock ties have been made offshore in the GoM and onshore in locations such as Texas and Louisiana or East/West of Present-day Mississippi river delta. The Aptian source rock is a moderate-high sulfur type of oil and composed of mainly Carbonates. This source rock is considered more “sour” and is located in southern Texas and South Florida Basin. The Tithonian is reported as the major source of oils for the GoM. This source rock is composed of carbonates which produce moderate to high sulfur content. These source rocks are dispersed throughout the basin's area. The vast amount of the source rock lies within the lowermost portion of the basins fill. Data varied in the report of thickness and TOC for each source rock but ranges were from 100 - 200 meters thick and 10 - 15 % TOC.
Traps are a combination of structural formation features that block upward migration of hydrocarbon. The Gulf of Mexico basin is very large and it contains a number of different traps. Some of the traps that can be found are: basement structural highs, primary salt domes and turtle anticlines, growth faults and roll-over anticlines, complex faults, folds and discontinuities associated with salt canopies, basin-floor fold and thrust belts, sub-unconformity truncation, facies changes, and terminations against salt.
In the GoM two extreme case of migration exist.
- Where no major listric (growth fault) systems occur, vertical oil and gas migration will be dominate.
- Where major listric (growth faults) systems occur a major cross-strata migration, component is introduced.
The majority of know fields and discovery in the GoM have migration resulting from cross-strata migration which results from either salt movement (diapers) or faulting (smear). Hydrocarbons are moved from deep source intervals to younger Tertiary reservoirs. Figure 3 Shows how the salt diaper has pierced the overlying sediments and created a trap and a migration pathway from older sources to newer ones.
Primary and Secondary Reservoirs
The oil fields are geographically dispersed throughout the GoM. In the United States, the GoM stretches across from East Texas to the Salt Domes of Louisiana, and the coastal plain from Texas to Alabama are also viable. The continental Shelf and Continental slope are all prolific petroleum provinces. Majority of the exploratory wells being drilled in the GoM are in the Miocene fan sands beneath the beneath the north-east Gulf. The reserves of these are 23 Bboe (Billion barrels of oil equivalent). Aside from these conventional sand plays, operators have drilled unconventional tight gas and shale reservoirs. Modern drilling and completion techniques have allowed these secondary reservoirs to be both economical and viable assets.The long history of deposition in the Gulf, there are multiple rock types. These range from dolomite and limestone and some very cemented sandstone and mudstone to unconsolidated sand and mud. The depositional environments from carbonate reefs to deep-marine submarine fans has provided many opportunities for potential reservoirs. Petroleum hydrocarbons have been explored and produced from every major stratigraphic unit from the Jurassic-age aeolian siltstones and Smackover-age shallow water limestones (Fig. 4). The porosity ranges from intergranular pores to secondary leached pores in deeply buried, highly cemented sandstones. Jurassic and Cretaceous carbonate reservoirs commonly exhibit fracture porosities. Fracture porosities are secondary porosity types that enhance the permeability of the reservoir. As conventional reservoirs have been exploited onshore, effort has successfully shifted to unconventional reservoirs, including fractured chalk (Austin), tight sand (Cotton Valley-Hosston), and shale (Bossier). Here, source rock and reservoir can be intimately mixed.
Future Assessment of the Basin
The Gulf of Mexico basin is a contradiction: it's a very mature petroleum basin but continues to yield new reserve plays. Over much of the basin's lifetime, operator's have only drilled has penetrated only the upper half to two-thirds of the sediment column. Exploring the basin's complex structures beneath the sandstone columns is in its infancy. Discoveries beneath the deep continental slope show signs for the presence of thick sand-rich units in the untested section. Deep-water Jurassic and Cretaceous areas of the eastern Gulf, remain largely not drilled, as does much of the continental slope on the western side of the gulf. Technological advances and changing energy economics, together with this vast untapped volume of demonstrably hydrocarbon-rich basin fill ensure that the GOM will continue as a major player for decades to come. Despite the steep growth of unconventional onshore oil and gas production, deepwater fields are projected to still be an important source for Global oil supply. GoM being the most prolific deepwater field in the world, will generate around 20% of the world's offshore production by 2030 (2 million barrels per day). The GoM is projected to supply the largest wedge of deepwater production in the future. There are several factors that causes this such as: an attractive fiscal regime (tax cuts in U.S), drops in break even costs, and technological/infrastructure advances in the basin.
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