Salt Plugs Research Articles

NaC1 crystallization at the MgCl2/NaC1 solution boundary–a possible natural barrier to the transport of radionuclides

AbstractConcentration, conductivity, temperature, and flow logs from sixteen brine-filled shafts in northern Germany have shown that the brines in all former potash salt mines exhibit a very sharp stratification into lower Mg-rich brine, an upper layer of Na-rich brine, and groundwater at the top. Laboratory experiments have shown that, at the MgCl2-brine/NaCl-brine boundary, both solutions become oversaturated with regard to NaCl, due to diffusion processes. NaCl therefore crystallizes from the solutions and forms a salt plug in the boundary region, which considerably reduces further diffusion. It is concluded that the observed effects would also take place in shafts. The backfilling material helps to nucleate the halite crystals and provides a structure on which they might be supported. The results of these experiments show that the density boundaries in the brine bodies act as barriers against transport of matter while the formation of a halite plug growing independently at the MgCl2/NaCl-brine interface acts as an additional barrier.

SIDESWIPE REFLECTIONS AND OTHER EXTERNAL AND INTERNAL REFLECTIONS FROM SALT PLUGS IN THE NORWEGIAN‐DANISH BASIN*

ABSTRACTSideswipe, foreswipe and backswipe reflections are described from salt plugs situated in the Norwegian‐Danish Basin (southern part of the Norwegian sector of the North Sea). These external reflections can be mapped and are shown to define the shoulders of a salt plug.Physical three‐dimensional models are described which aid in visualizing the various aspects of the external reflection process. Refracted waves which have been internally reflected in a salt plug have also been identified.

Geology and Exploration History of South Marsh Island Block 9 Field, Offshore, Louisiana: ABSTRACT

South Marsh Island Block 9 field, a piercement salt dome, is one of the older oil and gas fields in the Gulf of Mexico, yet significant exploration continues in the field to the present day. Through acquisition of new geological data and use of advanced geophysical techniques, recent exploration of this dome has more than doubled field reserves since 1980. The field is located 20 mi offshore in 60 ft of water. Domal growth occurred from Miocene through Pleistocene time; the salt plug truncates objective Pliocene and Miocene deltaic sands from 9,000 to 18,000 ft. Early exploration beginning in 1962 found limited reserves but established the presence of thick prospective sand-shale sequences. The dome was believed to be overhung, making definition of the salt-sediment interface difficult when using conventional seismic techniques. Through custom geophysical data acquisition and processing, Chevron was able to improve definition of the salt profile. Subsequent drilling discovered significant accumulations of hydrocarbons and led to successful second-generation drilling on previously tested and abandoned domal flanks. These new wells have resulted in the discovery of over 400 ft of hydrocarbons per well in multiple pay sands updip from previously drilled dry holes. This success, occurring 18 yr after initial testing of the dome, illustrates that significant new reserves may be found in old salt dome fields through the use of modern exploratory methods. End_of_Article - Last_Page 234------------

FISSION TRACK DATING OF AN APATITE CRYSTAL FROM HORMUZ ISLAND, IRAN

Geological evidence points to a Late PreCambrian origin for the Hormuz apatites associated with the red iron oxides, salt and other sedimentary and igneous rocks of the Hormuz Complex. Fission track dating of an apatite crystal from Hormuz Island gives an age of 55.4 ± 2.6 MM years for a cooling temperature of approximately 100d̀ C corresponding to the Paleocene‐Eocene boundary. This result is interpreted in the light of the burial and temperature history of the apatite associated with the Hormuz salt. It is concluded that the apatite clock registers the time elapsed since the temperature decreased to 100d̀ C at the Paleocene‐Eocene boundary. Steady cooling took place thereafter due to the continuing upward movement of the salt plug until Quaternary times. The stratigraphic position which the salt diapir (with the associated apatite) had reached 55 MM years ago corresponds to a level somewhere within the Triassic sequence. This would have been the zone of oil generation in Paleocene‐Eocene time, assuming that a source rock was present. Interpretational uncertainties are due to insufficient knowledge of the paleotemperature regime and of the stratigraphy and sedimentary thicknesses of the complete sequence down to the Precambrian. In addition, it is not possible to draw far‐reaching conclusions based on the dating of only one sample.

Growth of salt-induced structures in the Norwegian-Danish basin

Five relatively deep-seated salt plugs, situated in the Norwegian-Danish Basin, have been investigated. The Zechstein salt has pierced the Triassic and probably also the Jurassic strata in the area. In two of the structures salt has also intruded Lower Cretaceous strata. Otherwise, however, the relative upward movement of the salt has only resulted in the formation of salt-induced dome-shaped structures in the Lower Cretaceous and younger sediments overlying the salt plugs. Isopach maps, based on seismic reflection data, show that, as a rule, relative upward movement of the salt plugs has occurred during sedimentation. Such movements find their expression in local thinning of the interval, superimposed, as a rule, on a regional thickening. Using on-structure and off-structure thickness data, a quantity called the “convergence” can be calculated which numerically measures the proportional thinning of an interval over a given structure. The convergence values, thus calculated, show that there was continuous structural growth in all four structures that can be investigated in this way in Lower Cretaceous and later times except in Eocene times when there possibly was no growth at all in any of the four investigated structures. It is also found that a typical time-rate of growth during the post-Jurassic for these salt-induced structures is 1 mm per 1000 years (1 · 10 −3 mm/year) except for the Eocene where the rate of growth was possibly zero.

Salt tectonics in block [formula omitted] of the Norwegian sector of the North Sea

Seismic reflection data are used to investigate the structure of block 8 8 of the Norwegian sector of the North Sea, situated in the Norwegian—Danish Basin. Zechstein (Upper Permian) salt occurs in this basin, having given rise to widespread and intense salt tectonics. A number of reflections can be recognized and identified on the seismic sections. They are the Top Oligocene, the Top Paleocene, the Base Tertiary, and the Top Lower Cretaceous reflections, as well as a Jurassic reflection and the Base Permian Salt reflection (only found locally). The geological structure of the area is illustrated by means of seismic contour maps and cross-sections. Three salt plugs (a northern, a central and a southern one) are present in block 8/8. Two of these pierce Base Tertiary. The third (southern) one is more deep-seated. A major growth fault connects the northern and central plugs. The southern salt plug is surrounded by a rim syncline. The movements on the major fault and in the salt plugs in the course of geological time are investigated. It is found that there is evidence for movement in Late Jurassic, Cretaceous and Tertiary times until at least Early Miocene times. The possible cause of the structure of block 8 8 is considered in detail. It is found that the movement of the salt in at least the central salt plug is intimately connected with movements on the major growth fault. The hypothesis is advanced that all local tectonic movements are due to the flow of salt and a scheme for this salt flow is proposed. A number of special features of the tectonics of the area which support this concept are dis- cussed. Volumetric considerations are also used. The approximate volume of the salt in the plugs as well as the volume of salt drained from part of the area are calculated. Finally, deep reflection evidence is used to estimate the depth of the base of the Zech- stein salt and the top of the crystalline basement in the southern part of the area.

THE EMERGENT HORMUZ SALT PLUGS OF SOUTHERN IRAN

The extrusive salt plugs in the “gently folded zone”of the Zagros mountain belt are almost exclusively composed of Hormuz evaporites carrying exotic blocks, originating at depth of 3–5 km below surface, dated as mainly late Precambrian.In recent years, there have been considerable advances in the understanding of the history of emplacement, on the stratigraphy of the very large exotic blocks of bedded Infra‐Cambrian sediments, on the stratigraphy of the very large exotic blocks of bedded Infra‐ Cambrian sediments on the entrained volcanic and intrusive igneous rocks and on the mineral occurrences. The charactreistic and controversial feature are described, with the minearl occurrences. The characteristic and controversial fetures are described, with emphasis on the newer information and on the relationships of the plugs to normal folds, thrusts and transverrse fracture zones.The absences at surface of the many cubic kms of country rock displaced from the neck of each plug has hitheto presented an unsoloved probelm; prevoious explanation fail to fit availabel facts. It is now postulated that in most cases initial extrusion took place at higher stratigraphic levels, and that the original vent material has been eroded along with the sediments lost following uplift of the mountain belt.The concept that pre‐orogenic plug movement was not exceptinal but normal, so that the plugs penetrated an essentially flat‐lying sedimentary blanket, resolves also the contrast between their regular distribution in the southeastern Zagros and their random reltion to the Pliocene fold trends, as well as the tendency for inflexions of fold axes to be located where they encounter the weak points provided by the salt intrusions.

THE EMERGENT HORMUZ SALT PLUGS OF SOUTHERN IRAN

THE EMERGENT HORMUZ SALT PLUGS OF SOUTHERN IRAN

Curved Well Conductors and Offshore Platform Hydrocarbon Development

This paper discusses drilling offshore directional wells with conventional equipment through curved conductor pipes. Curves and graphs are presented to compare the lateral-reach characteristics of directional wells drilled through curved and vertical pipes. Well trajectory, directional drilling data, and tubular information illustrate the success achieved by drilling wells through curved conductors. Introduction The 1970 Federal Offshore Louisiana Lease Sale required the development of shallow-depth hydrocarbons from sites in 150 to 400 ft of water. Because the lateral reach of a conventional vertical conductor was limited in depths ranging from 3,500 to 6,000 ft, several platforms would be required to handle these requirements. The potential for extended lateral reach prompted a review of potential for extended lateral reach prompted a review of unconventional methods to improve platform well reach. (The term, conductor, used in this paper refers to the part of the well tubulars often called drive pipe.) Fig. 1 illustrates the reach improvement obtained when the rate of building drift angle increased from 2 degrees to 4 degrees per 100 ft (55 degrees maximum). If an angle builds faster below the per 100 ft (55 degrees maximum). If an angle builds faster below the conductor, the coverage of vertical conductors is improved noticeably, as evidenced by the area factor values in Fig. 1. Initiating well deflection above the water instead of below the conductor also improves the reach significantly. In discussing the problem with drilling engineers, we found that a well could be drilled around a curve of 6 degrees per 100 ft if the curvature was smooth and drillstring weight below the curvature minimized. This rapid increase in drift angle through the 150- to 400-ft water zone of an offshore platform can improve the mud-line drift angle 9 degrees to 24 degrees, respectively. When an angle improvement above the mud line is combined with that below the mud line, the curved conductor becomes a very useful tool. This method was evaluated with a successful field-test well in 1970. Well Planning Considerations Generally, curved conductors are most advantageous at water depths of 150 ft when developing hydrocarbon reservoirs shallower than a true vertical depth of 7,000 ft. These conductors have applications not only in conventional rig development drilling, but also in exploratory drilling from platforms with conditions that normally could require the more expensive mobile rigs. The improved reach characteristics of curved conductors has a significant impact on the choice of platform sites. Locations may be selected at some distance from the center of hydrocarbon accumulation to avoid surface and near-surface obstructions such as salt plugs, shallow faults, sea-bed reef deposits, gas seeps, shallow gas pockets, mud slides, debris, and shipping fairways. Final pockets, mud slides, debris, and shipping fairways. Final choice of platform location also is influential when considering the most efficient reservoir development and optimization of the drilling and completion techniques. The design of an individual well trajectory is influenced by reservoir size, required maximum lateral well displacement, and drilling or completion considerations. Drilling and completion efficiency generally is reduced with an increasing wellbore angle and by tong sections with varying hole angle. The curve conductor can minimize these problems by reducing the length of wellbore with varying angle and the maximum drift angle required to reach a target. JPT P. 440

Possible Igneous Analog of Salt Dome Tectonics, Clark Mountains, Southeastern California: GEOLOGICAL NOTES

A small dioritic pluton and an associated marble sheath have been diapirically emplaced into deformed lower Paleozoic carbonate rocks. The diapir appears structurally analogous to intrusive salt plugs with associated shale sheaths. Possibly the emplacement of the diapir, as that of salt plugs in the Gulf Coast area, was in response to buoyant forces.

Evolution of Belle Isle Salt Dome, Louisiana: ABSTRACT

The Plio-Miocene subsurface geology of Belle Isle dome in coastal Louisiana was studied using electrical logs. The pattern of syndepositional folding, faulting, and sand distribution at this shallow-piercement salt structure reflects variations with time in the configuration of the paleotopographic mound and indicates contemporaneous intrusive movement and sedimentation. Intrusive growth proceeded from an elongate shale and salt mass (ridge stage) to a buried cylindrical salt plug (deep-plug stage) to a plug which maintained a near-surface position (shallow-plug stage). Each growth stage has a particular style of folding and normal growth faulting that is related to the shape and burial history of the intrusive. The ridge stage is characterized by movement on tangential faults and intrusive-coincident peripheral faults. The highly convergent beds of the deep-plug stage record the main episode of radial faulting. Shallow-plug-stage sediments are rarely faulted, and have low-sedimentary convergence rates. During both plug stages the salt-sediment interface acted as a vertical circular fault. The shallow-plug stage is associated with slowly deposited nonmarine sediments, whereas the deep-plug stage is associated with rapidly deposited deltaic sediments. It is postulated that deposition rates largely determined burial depth of plug and that changes in absolute rate of salt movement during plug stages were minor. Because of the large topographic mound present during the ridge stage and deep-plug stage, sandstones grade to shale near the salt, thereby creating numerous stratigraphic traps. End_of_Article - Last_Page 1835------------

La restitution géométrique des profils sismiques continus

During continuous seismic profiling in deep sea, the reflecting points on sloping reflectors occur outside of the vertical through the ship. The formulas which give the values of the offset are given, so facilitating the transference from the plotted time-distance diagram to a geological section. These formulas are: ( 1) lateral offset ξ is up-dip and equal to Σ( t i V i 2) A; ( 2) vertical offset τ is toward earlier times and equal to 1 2 Σ(t i V i 2)A 2 ; where A = sin i V . It is shown that, if the reflector is curved, as in a syncline, the plotted diagram may show cusps; for an anticline, the line of these cusps can be the border of an area without reflections, which should not be mistaken for a salt plug.

Salt Piercement in Shallow Gulf Coast Salt Structures

Data from 16 salt structures of the Gulf Coast province have been analyzed to (1) document some of the piercement features on shallow Gulf Coast salt structures and (2) investigate some of the mechanisms and controls of the salt-piercement processes. The process of piercement by vertical displacement of overburden by salt has been the primary subject of this study. Accordingly, the investigations have concerned mainly the upper section of alternating sandstone and shale beds that are in contact with the salt and show a discordant relation with the salt core. The two general types of piercement processes recognized on shallow Gulf Coast salt domes are (1) piercement by rupture of the overburden through fracturing, faulting, and tilting of the overburden upward and outward in segments, with rise of the salt being enhanced by erosion, and (2) piercement by vertical displacement of overburden ahead of the growing salt mass, accompanied by peripheral faulting surrounding the salt dome. The discordant relation commonly observed in the upper parts of shallow salt structures between the salt plug and the enveloping strata is the result of piercement by vertical displacement of overburden by salt. The discordant relation can be produced by piercement of each bed contemporaneously with deposition or by postdepositional piercement of a thick section of overburden. Postdepositional piercement and vertical displacement of overburden as much as 2,400 ft thick can be documented in the Gulf Coast. Piercement by vertical displacement is most likely where the salt has a plug shape and is growing by a process of plug flow. The plug shape evolves from a broad salt mound or pillow in the later stages of salt dome growth. Piercement by rupture and outward tilting of segments of overburden is most likely in the earlier stages of salt-dome growth, when the salt mass is in the broad mound or pillow shape or is changing from the pillow to the plug shape.

Geochemistry of pore waters from Shell Oil Company drill holes on the continental slope of the northern Gulf of Mexico

Pore waters were analyzed from 6 holes drilled from M.V. “Eureka” as a part of the Shell Oil Co. deeper offshore study. The holes were drilled in water depths of 600–3,000 ft. (approximately 180–550 m) and penetrated up to 1,000 ft. (300 m) of Pliocene-Recent clayey sediments. Salt and anhydrite caprock was encountered in one diapiric structure on the continental slope. Samples from holes drilled near diapiric structures showed systematic increases of pore-water salinity with depth, suggestive of salt diffusion from underlying salt plugs. Anomalous concentrations of K and Br indicate that at least one plug contains late-stage evaporite minerals. Salinities approaching halite saturation were observed. Samples from holes away from diapiric structures showed little change in pore-water chemistry, except for loss of SO 4 and other variations attributable to early-stage diagenetic reactions with enclosing sediments. Thus, increased salt concentrations in even shallow sediments from this part of the Gulf appear to provide an indicator of salt masses at depth.

Permian Evaporites in North Sea Basin

Thick evaporite beds are present in the Permian beneath the North Sea, and can be correlated with the known successions in Germany and in England. The evaporites are more widespread in the upper division (Zechstein) than in the lower (Rotliegendes). This paper concerns chiefly the Zechstein of the English side of the basin, in which four main evaporite cycles are present. Zechstein salt movement began near the end of the Early Triassic when the overburden was only about 2,000 ft (610 m) thick, and continued throughout the Mesozoic and Tertiary. Movement was earlier in the west than in the east. Salt plugs are associated particularly with the margins of a large NNW-SSE trough which became fully developed during the Jurassic. Important structural features and thickness changes in the Mesozoic and Tertiary rocks were caused by the salt movement, and are almost entirely compensated by salt at depth. As a result, the base of the Permian is nearly parallel, on a regional scale, with the base of the Quaternary though separated from it by about 10,000 ft (3,000 m) of moderately complicated strata.

Fully saturated brines

This type of subsurface water contains more solutes than solvent and is found only at depths exceeding 1000–3000 m, where it exists at high pressures and temperatures, in the form of a dark heavy liquor in rocks ranging in age from Lower Cambrian to Upper Jurassic. The eight specimens of such brines here reported are characteristically devoid of SO4 and HCO3, with K > Na and even K > Mg, at pH 2.8 to 4.6, containing up to 9 g of Br and up to 4 g Sr/lit; the Br:1 ratio is extraordinarily high and the brines are supercharged with H2S. They may be interpreted as end-products of concentration and metamorphism. Such brines, when tapped, e.g. by drilling for hydrocarbons, produde powerful salt plugs in drillholes, capable even of stopping an oil gusher, as their solutes (up to 650 g/lit) are precipitated rapidly at reduced pressures. – IGR Staff.

Permian Evaporites in North Sea Basin: ABSTRACT

Thick evaporites are present in the Permian beneath the North Sea, and can be correlated with the known successions in Germany and in England. The evaporites are more widespread in the upper division (Zechstein) than in the lower (Rotliegendes). This paper deals chiefly with the Zechstein of the English side of the basin, in which four main evaporite cycles are present. Zechstein salt movement commenced near the end of the Early Triassic when the overburden was only about 2,000 ft, and continued throughout the Mesozoic and Tertiary. Movement was earlier in the west than in the east. Salt plugs are associated particularly with the margins of a large NNW-SSE-trending trough which became fully developed during the Jurassic. Important structural features and thickness changes in the Mesozoic and Tertiary rocks originated as a result of the salt movement, and are compensated almost entirely by salt at depth. As a result, the base of the Permian nearly is parallel, on a regional scale, with the base of the Quaternary though separated from it by about 10,000 ft of moderately complicated strata. End_of_Article - Last_Page 522------------

OUTLINE GEOLOGY OF THE SOUTHERN NORTH SEA BASIN

Summary The address outlines the geological results of exploration for natural gas in the southern North Sea Basin as derived from geophysical surveys and deep drilling operations. The greater part of the marine area is stratigraphically comparable with the landward continuation of the basin in northern Germany. Marine Tertiary is well developed; the Trias includes a representative of Muschelkalk and the Permian is bipartite, with a Zechstein including thick evaporites overlying the Rotliegendes. True folding is very gentle, deep-seated folds and faults trending predominantly north-west–south-east (in the west) and north–south (in the east). A major phase of movement occurred in pre-Aptian times, resulting in widespread transgression of late Cretaceous on to early Jurassic and Triassic rocks. The dominant tectonic feature is the widespread development of halokinetic structures due to movement of Zechstein salt, the forms varying from pillows to diapiric salt plugs. This movement began in the Triassic and continued through Jurassic, Mesozoic and Tertiary times.

Evidence for Deep Salt Deposits in Western Florida Panhandle

Salt intrusions derived from the Louann Salt of Jurassic (?) age are abundant in the Gulf coastal plain and are found from Texas to Alabama. Evidence suggesting that the Louann Salt may extend into the western Florida Panhandle includes the following. 1. The Mississippi Interior Salt Dome basin strikes directly toward the western Florida Panhandle: the easternmost proved salt dome in this basin is only 32 miles away. 2. Small round topographic highs occur on the continental shelf off the Florida coast; some of these may be related to salt domes. 3. Seismic-reflection measurements reveal the presence of two anticlinal structures, one very broad, the other dome-like, on the continental shelf south of the western Florida Panhandle; there is a strong possibility (not yet proved) that these are related to salt domes or ridges. The broad structure is cut by graben faults. 4. A recently discovered domal structure broken by a northwest-trending graben is centered under the town of Jay in Santa Rosa County. The faults are similar to those that occur over known salt domes in Texas and Louisiana. 5. The faults making up the Pollard graben, which extends from Alabama into northern Santa Rosa County, Florida, are of a type that could have been caused by repeated flowage of salt resulting from sedimentary loading during subsidence of the Gulf Coast geosyncline. These faults are contemporaneous; that is, the faulting took place during deposition of the beds that are now displaced. The faults are characterized by: (a) numerous small offsets that occurred at least from Late Cretaceous through middle Oligocene time; (b) increasing throw with depth; and (c) thicker downthrown beds compared with the correlative upthrown beds. Currie (1956) has demonstrated that grabens composed of contemporaneous faults may be produced by upward-moving salt plugs. 6. Gravity minima which may be caused by salt domes are present in westernmost Florida and southwestern Alabama. The three deepest oil test holes in Escambia and Santa Rosa Counties, Florida, were drilled to depths of about 12,500 feet and reached total depth in the Hosston Formation of Early Cretaceous age. An oil well in the Citronelle field, Alabama (about 38 miles northwest of Florida) reached the Louann(?) Salt at about 19,000 feet. Thus, if the salt is present in the western Florida Panhandle, it probably is at least 6,000 feet deeper than the greatest depth so far reached by drilling.

Extrusive Shale Masses: New Gulf Coast Exploration Frontier: ABSTRACT

Many Gulf Coast masses are extrusive deposits formed by the processes of sedimentary volcanism. Sedimentary volcanic deposits have been recognized only recently in Tertiary strata of the Gulf Coast. Diagnostic evidence for this phenomenon is found at outcrops of the Catahoula Formation (middle Tertiary) in the south Texas counties of Live Oak, McMullen, Duval, and Webb. The absence of active sedimentary volcanism in the Gulf Coast and the difficulty of recognizing this phenomenon in ancient rocks are causes for a general omission of this subject from the American geologic literature; consequently, explorationists are overlooking diapiric and possibly extrusive origins for numerous Gulf Coast masses. The ultimate relation of a buried extrusive mass to adjacent and overlying beds is determined by the amount of mudflow buildup and preservation during the time of deposition of the nearby normally deposited beds. If the sum of mudflow deposition (with accompanying erosion) greatly exceeds the sedimentation of the adjacent beds, large mudflow domes and ridges may form prominent topographic features. Conversely, if the rate of sedimentation of adjacent beds equals or exceeds that of the mudflow accumulation, an ill-defined mudflow facies is formed. Most thick, extrusive bodies probably are composite masses of both rapidly and sporadically extruded mudflows interfingered with normally deposited beds. Growth of an extrusive dome is attained by a sequence of mudflows extruded from clusters of mudcones. Dips of mudflow layers increase as each succeeding layer is extruded and a domal topographic feature forms. Slopes of active mudcones are commonly 30-40°, depending on the mud viscosity; cones are known to exceed 1,500 ft. in height. Commonly, mudflows range in thickness from several inches to 50 ft. and extend as much as 2 mi. from their parent vent. Mudflow extrusions may take place simultaneously for many miles along a fault system. Active mudflow ridges 20 mi. long are known in West Pakistan. These flat-topped ridges are hundreds of feet thick and have steep sides with 40-70° slopes. Erratic rocks commonly are brought up thousands of feet stratigraphically by mudflows. Erratics up to 3 ft. in maximum dimensions are common, and rarer occurrences of blocks with 50-ft. dimensions are known. Microfossils, thousands of feet out of place, occur in many places within extrusive mudflows or masses. Diagnostic evidence of diapiric clastic rocks includes: erratic fossils, churned pellets, gas bubbles, and disrupted rock frameworks. Revised exploratory thinking is required to search successfully for and to recognize subsurface diapiric masses. Diapiric masses are formed in specific basins, along certain trends, and during favorable geologic times. Although intrusive plugs exhibit the same pronounced structures as salt plugs, buried extrusive masses are not associated generally with pronounced radial faulting, sharply upturned End_Page 2324------------------------------ beds, or other commonly recognized structural attributes of intrusive masses. Diapiric shales produce negative gravity anomalies because of low densities. Density logs show densities to be almost as low as salt. Low velocities (indicated by sonic logs) cause shale-mass structures to be mapped seismically as lows instead of highs, unless correct velocity functions are used. A common clue to subsurface diapiric masses is 0.5-ohm resistivity (IES log) caused mainly by high water content of the shale. Few correlations, if any, can be made in the diapiric mass. An abnormal microfaunal sequence is found in nearly every case, as is high-pressure gas. Because of their greater magnitude and distinguishable direction, mudflow dips within an extrusive mass can be recognized commonly by a dipmeter survey. Dips recorded within an intrusive plug or a shale sheath should be random in both magnitude and direction. Sidewall cores within a diapiric mass contain churned pellets and gas bubbles in the units and also bear disrupted sand-grain frameworks in the sandstone bodies. Sandy, water-filled, gas-churned mudflows are high-porosity, low-permeability masses that serve as barriers to hydrocarbon migration. Intrusive structures must have had a timely injection in order to trap migrating hydrocarbons whereas extrusive masses are unusual barriers because the barrier is present before or during deposition of the adjacent beds. Systematic recognition and delineation of extrusive masses in the Gulf Goast by both conventional and improved exploration methods will open new frontiers to Gulf Coast petroleum exploration. End_of_Article - Last_Page 2325------------