HOW OIL SEEPS, DISCOVERIES RELATE IN DEEPWATER GULF OF MEXICO

Roger Sassen, James M. Brooks, Mahlon C. Kennicutt II, Ian R. MacDonald, Norman L. Guinasso Jr.
Geochemical and Environmental Research Group
College Station, Tex.

Biomarker compounds of crude oils from deepwater Gulf of Mexico seeps are consistent with an origin from deeply-buried Mesozoic carbonate source rocks. The known oil reserves, however, are trapped in shallow Miocene to Pleistocene sands.

Several kilometers of vertical migration must be invoked to explain the presence of crude oil from deep sources in shallow reservoir sands. Salt-related fractures and faults serve as efficient conduits for vertical migration through the thick sedimentary section.

Thermal history models suggest that oil migration off Louisiana started during the Miocene and continues at present in some areas.

Given the importance of vertical migration in the deep gulf, it should not be surprising that much oil migrating from source rocks bypasses traps and reaches the sea floor. Moreover, oil also leaks after emplacement in traps because no seal is perfect in an area of rapid sediment loading that influences salt and fault movement.

Macroseeps are localized areas on the sea floor, tens or hundreds of meters across, where transport of oil and gas to sediments is so rapid that the destructive effects of bacterial oxidation cannot keep pace.

Over time, oil accumulates in sediments. Free gas is present. Both oil and gas leak to the water column episodically. Gas also accumulates in deep, cold sea floor sediments as ice-like hydrates.

Bacterial oxidation of oil and gas produces carbon dioxide that precipitates as authigenic carbonate rock.

Life on the floor of the deep gulf, as in deep water areas elsewhere, should be sharply limited by lack of organic nutrients. However spectacular life oases dominated by unusual chemosynthetic organisms occur where hydrocarbon-based bacterial activity creates a favorable life environment.

Chemosynthetic tube worms and other organisms that oxidize hydrogen sulfide were first discovered at deep-water hydrothermal vents in the Pacific. In contrast to the organisms first discovered in hot rift environments, the deep gulf communities are found at low temperatures.

Knowledge about deep gulf macroseeps has already influenced exploitation decisions. For example, the seep-related chemosynthetic communities are of environmental interest because of their close association with oil and gas fields, as well as with future dig sites.

It is now routine to search for chemosynthetic communities and oil-stained sediments before drilling in many areas of the deep gulf. The presence of gas hydrates in sediments constrains engineering plans for deepwater production platforms. In addition, sea-floor outcrops of carbonate rock affect plans for routing pipelines.

This article presents new data on deep gulf macroseeps and their association with oil discoveries. The deep gulf case history shows how information on seeps can affect exploration decisions. Insights from the deep gulf can be used to constrain risk in exploration of other offshore basins.

SEEING SEEPS

Although large numbers of piston cores have sampled hydrocarbon seepage in the deep gulf, the most meaningful way to understand seeps is to see them from a research submarine.

Geochemical and Environmental Research Group (GERG) and oil company researchers have made numerous dives in the deep gulf to study sea floor features related to hydrocarbon seepage.

Linear arrays of seeps occur along sea-floor fault traces. Dark oil-stained sediments can be seen and sampled along steeply-dipping fault scarps, and near fault vents in outcropping masses of carbonate rock. Gas bubble trains that issue continuously or intermittently to the water column are often observed during submarine dives. Active gas seepage has been photographed on the deep gulf floor (Fig. 1).

Deep gulf sediments experience cold temperatures as well as high pressures from the overlying water column. Gas hydrates form when free hydrocarbon gas migrates from depth to a fractured or permeable zone in the cold sediments.

Conditions favorable for hydrate formation generally exist at water depths greater than 400 m in the gulf.

Hydrates occur in sediments as orange or white nodules and as discrete layers. Recent research submarine operations in the deep Green Canyon area confirm that gas hydrates outcrop on the sea floor.

Some sea-floor features are formed directly as a consequence of rapid fluid migration. For example, mud volcanoes are spectacular seepage sites associated with faults and shallow overpressure. Mud volcanoes episodically discharge gas, oil, and brines to the water column.

A miniature mud volcano in Green Canyon was recently observed releasing gassy brines that flowed downslope from a small pockmark at its apex (Fig. 2). The volcanoes are thought to be constructional features created by settling of sediments initially suspended in the water column by rapid fluid movement .2

Transport is sometimes so rapid that oil entering the water column from mud volcanoes is not biodegraded.

Natural oil slicks are observed on the sea surface over mud volcanoes and other sea floor features. Continuing research at GERG shows that natural oil slicks are much more common in the Gulf of Mexico than previously thought.

NASA imagery from orbital platforms, including the space shuttle, provides new insight to the distribution of oil slicks in the gulf. Oil slicks appear as light-colored bands on the sea surface in imagery of the Green Canyon area off Louisiana (Fig. 3).

Unlike mud volcanoes, which are constructional features, pockmarks and craters are the result of episodic sea floor erosion by brines and gases migrating up faults in areas of shallow over pressure.

Pockmarks are frequently observed from research submersibles. They are sometimes floored with authigenic carbonate rubble and expose sea-floor sediments in their flanks that have been cemented with authigenic carbonate.'

The volume of seep-related authigenic carbonate rock that outcrops over salt and near faults in the deep gulf is immense. 3,4,1 Most of the carbonate is calcite and aragonite, but some dolomite has also been identified.

The carbonate records the migration of oil and gas over long spans of time. Bacteria have slowly oxidized hydrocarbons to form carbon dioxide that precipitates as the carbonate rock. The carbonate rock inherits the light carbon isotopic composition ( 13C) of the hydrocarbon starting materials.

Outcropping carbonate rocks on the sea floor are sometimes encrusted with deep-sea organisms such as crinoids (Fig. 4).

At present, about 50 seep-related chemosynthetic communities have been identified in the deep gulf. This is a minimum number since there has been no gulf-wide search for chemosynthetic organisms.

Colonies of large tube worms (l m in length) with bacterial symbionts that oxidize H2S have been documented in 290-2,200 m of water in Green Canyon, Garden Banks, Viosca Knoll, and Alaminos Canyon.6 The tube worms sometimes grow in bushlike aggregates at oil seeps (Fig. 5).

Seep mussels with bacterial symbionts that utilize methane are common.' The mussels sometimes occur together with tube worms and other organisms (Fig. 6). Two distinct assemblages of dams with bacterial symbionts that oxidize H2S have also been described.8

DEEP GULF OVERVIEW

Mapping oil seeps across the deep gulf defines areas with low source risk. For example, piston cores and later research submersible operations mapped oil seepage and chemosynthetic communities in extremely deep water of Alaminos Canyon near the edge of the Sigsbee escarpment. This information encouraged lease acquisition and should eventually encourage drilling in that part of the deep gulf.

Data on several thousand piston core samples of sea-floor sediments taken by GERG since 1983 define the limits of the deep gulf play fairway ahead of the drill bit.

In the eastern gulf, oil seeps have been sampled along the Florida escarpment. Seeps extend westward in a wide belt across the continental slope off Louisiana and Texas.

Seeps off Louisiana and Texas cluster around the edges of salt-basin "cooking pots" on the upper continental slope -where the early discoveries were made - south to the edge of the Sigsbee escarpment in extremely deep water. Seeps are also found near oil fields as far south as the Campeche shelf off Mexico.

Some areas in the deep gulf lack meaningful oil seepage because gas is the main hydrocarbon type in subsurface reservoirs. Source rocks in such areas are now burned out for oil generation. Total scanning fluorescence and other geochemical data on cores can be used to map the regional and subregional distribution of oil versus gas-condensate or gas.'

SEEPS AND FIELDS

Calibrating the presence and absence of macroseeps and chemosynthetic communities with known discoveries is instructive.

Most large traps in the deep gulf are marked by macroseep features located on geophysically obvious migration conduits from depth to the sea floor. For example, oil seeps and chemosynthetic communities are documented over shallow salt and faults in the vicinity of the deepwater Auger, Cooper, Jolliet, Popeye, Vancouver, and Ram-Powell finds (Fig- 7).

Oil seeps also occur in the vicinity of the Mars and Bull-winkle discoveries.

It should be emphasized that the presence of macroseeps does not guarantee success, and it is possible to drill dry holes in seep areas for any number of reasons. The wrong play concept can result in a missed opportunity.

For example, a Pleistocene bright spot may not be the best exploration objective in an area where oil charge occurred during the Miocene. Overall, using information on seeps in a prospect-focused mode offers a meaningful statistical edge in making drilling decisions.

CASE HISTORIES

Jolliet field in northwest Green Canyon and the Cooper discovery in northeast Garden Banks are two examples of the association between macroseeps and deep gulf structural traps that are worth describing in detail.

Jolliet field in Green Canyon (GC) blocks 184-185 is a salt-related thrust-fault trap with medium-gravity crude oil in Pleistocene turbidite sands.'o A cross section illustrating the main geologic features of Jolliet field also simply defines the main areas of oil seepage (Fig. 8).

Jolliet field leaks rapidly along salt and fault conduits to the sea floor. Oil from sea-floor sediments, as well as from sea-surface oil slicks and tar balls, have been geochemically fingerprinted to oil produced from reservoirs of Jolliet field."

The GC 140 salt diapir (Fig. 8) has been at shallow depths near the sea floor for much of the Pleistocene, and sediments over the diapir are heavily faulted." Most oil seepage and development of shallow carbonate probably occurred during the Pleistocene.

Outcropping authigenic carbonate rock is mapped at 230-530 m water depths across GC 140, directly north of Jolliet field. 3 The discontinuous outcrop of authigenic carbonate is 4 km across with relief up to 10 m. Carbonates are present with a wide range of light 13C values (-16.6 to -53.9 0/00 PDB) from bacterial oxidation of oil and biogenic methane.

Gas bubble trains and chemosynthetic bacterial mats have been observed during research submersible dives to the GC 140 outcrop area. One bubble train sampled in the water column just above a sea-floor vent consisted of biogenic methane with a 813(f of - 66.3 o/o.. 12

Although some oil is present at the sea floor, piston cores indicate that the carbonate outcrop zone is not presently a major site of thermogenic macroseepage. Small clusters of tube worms nevertheless survive within sheltered fault vents in carbonate rock at 290 m water depth.'

In contrast to the low rates of seepage presently observed in GC 140, high rates of seepage occur near faults on the eastern border of GC 184 (Fig. 8).

Piston cores targeted on a major growth fault in GC 184 resulted in the first discovery of 1 thermogenic gas hydrates. The hydrates were orange in color and associated with oil-stained carbonate rubble. All previous discoveries of gas hydrates had consisted solely of biogenic methane.

Bush Hill in GC 185 is a fault-related sea-floor mound with about 40 m of relief at 530-570 m water depths 6 (Fig. 8). The Bush Hill site is characterized by spectacular colonies of chemosynthetic tube worms and mussels.

The tube worms cluster at oil seeps where abundant hydrogen sulfide is present, whereas the mussels cluster at thermogenic methane seeps.' Sediment cores from Bush Hill show oil staining, gas hydrates, and authigenic carbonate. 2

The Cooper discovery in Garden Banks (GB) 388 is associated with macroseeps and chemosynthetic communities over fractured shallow salt in 670-700 m water depths. 14 The sea floor displays irregular topography with fault scarps, pockmarks, and outcropping authigenic carbonate rock.

Piston cores from GB 388 and vicinity are characterized by visible crude oil and gas pockets, as well as hydrogen sulfide. Gas hydrates are also present.

Chemosynthetic organisms at the GB 388 site include tube worms and seep mussels. A number of other macroseep localities with chemosynthetic organisms are identified over shallow salt ridges in the vicinity of GB 388.

FRONTIER BASIN

Lack of hydrocarbon charge is the biggest single risk in exploration of frontier basins.

The deep Gulf of Mexico is the simple case where the link between oil seeps, play fairways, and discoveries can be documented. The value of the deep gulf case history is that it shows how information on seeps can be used to impact exploration for oil in other offshore areas.

It should be stressed, though, that there are many differences between the deep gulf and other offshore basins. The intensity of seepage can be greater or lesser than in the deep gulf example. This is because of basic differences in geology that control rates of seepage to the sea floor, as well as differences in source rock type and thermal history that control the timing of oil migration.

Effective interpretation of seep data requires a multidisciplinary team of geologists, geophysicists, and geochemists. the key is to use regional geophysical profiles to select piston core locations over likely seepage conduits near the sea floor, such as salt or shale diapers, faults, or stratigraphic pinchouts.

Unambiguous predrill evidence of an active petroleum system helps constrain risk in frontier basins.

Analysis of oil recovered from piston cores provides key information on source rock types and thermal maturity history that otherwise is difficult to' obtain ahead of the drill bit.

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