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Variation in Salt Dome Faulting, Coastal Salt Basin
Many of the major growth faults affecting diapiric structures (salt domes and clay or "shale" domes) originate in the upper continental slope environment. Faults flank diapiric ridges which subsequently segment into individual diapiric structures as continental shelf deposition commences. Upper slope sand depocenters are often associated with these growth faults. Some upper slope faults remain active as continental shelf and alluvial plain deposition continues. Others become inactive and are buried. The resulting fault patterns vary with respect to the type of diapiric structure on which they occur.
Diapiric structures can be classified by the relationship between the diapir top and the objective section (continental shelf sands and shales and sandy upper slope strata make up the objective section):
Penetrant (or shallow piercement) salt domes: salt pierces the entire objective section plus much or all of the overlying nonobjective alluvial strata. Many diapirs unburied and still active.
Semipenetrant (or intermediate piercement) salt domes: salt pierces part but not all of the objective section, arching the shallower overlying beds. Diapir buried and inactive.
Nonpenetrant (or deep-seated) diapiric structures: diapir (salt, clay) is buried in upper slope shales beneath the base of arched objective section. Diapir inactive.
Fault patterns on Coastal Salt Basin diapiric structures fall into three basic classes:
Single or multiple offset: one or more semiparallel faults downthrown in the same direction.
Compensated: two or more semiparallel faults downthrown in opposite directions, forming grabens or horsts.
Crossed offset: two or more faults in crossed orientation.
Over 200 salt domes in the Coastal Salt Basin (offshore and onshore south Louisiana and southeast Texas) were analyzed for fault type, orientation, and pattern. Significant variations and trends in fault characteristics occur by salt dome type, and are illustrated graphically. While most diapiric structure fault patterns fall in one of the three basic classes, combinations of two patterns are relatively common on certain types of salt domes. Characteristic fault orientations and patterns occur repeatedly with each salt dome type. These data should be useful to salt dome workers interested in diapiric structure development and in origination of drilling prospects on mature and undeveloped salt domes.
Coastal Salt Basin (South Louisiana and Houston Embayment Salt Basins) salt domes are characteristically heavily faulted. Does this faulting occur randomly, or does it vary in a consistent manner, especially with respect to salt dome type? Analysis of fault type, pattern and orientation on over 200 salt domes confirms the latter. While faulting on heavily drilled salt domes often appears bewilderingly complex, the major growth faults usually occur in coherent, classifiable and reoccurring patterns. The fault patterns often provide important clues to the history of salt dome development. They can also be used as a guide for deep flank exploratory drilling, as major salt dome faults usually control the occurrence of upper slope sand-rich depocenters.
Many salt dome growth faults first form in the continental slope environment. On the upper slope, they often occur as peripheral growth faults formed along the flanks of numerous topographically high ridges with cores of diapiric materials, as shown (Figure 1). Depocenters are localized along the diapiric ridge flanks. These depocenters trap upper slope sands, especially in seafloor lows resulting from salt withdrawal and located downthrown to peripheral faults flanking the ridges. As the more rapidly deposited, sandy continental shelf strata prograde basinward and bury the upper slope, the diapiric ridges segment into individual salt domes of various types. Many of the ancestral peripheral faults formed on the upper slope remain active during subsequent deposition. They affect petroleum accumulation and often play a major role in salt dome structural development. The distribution of sands prospective for petroleum around and above diapiric salt is influenced by ancestral faults. However, some ancestral faults
become inactive and are buried in the outer shelf environment. Some originally flanked upper slope depocenters and may control unrecognized deep exploration targets. Younger growth faults which also affect petroleum accumulation form during deposition of continental shelf strata. Faulting diminishes and dies out within the overlying shallow continental sequence, which usually contains few significant accumulations.
SALT DOME CLASSIFICATION
Deltaic, delta fringe, continental shelf and sandy upper slope sediments usually contain most of the oil and gas in salt dome fields and therefore constitute the objective section. The underlying shales and overlying alluvium are rarely productive and are considered non objective. Salt dome geologists are most interested in the interrelationship of the objective section and the salt diapir. Sand distribution, faulting and petroleum occurrence all depend upon this interrelationship. Because of this, various schemes of salt dome classification have evolved over the years among salt dome geologists. The classification used herein is defined and shown in Figure 2.
FAULT TYPE CLASSIFICATION
A useful fault type classification developed by G.D. O'Brien (1961) is shown in Figure 3. The growth fault type names describe the fault to diapir and diapiric structure relationship at objective section levels. Fault orientation is very important as well, especially when related to salt dome type.
Commonly, many smaller growth faults occur on penetrant and semipenetrant domes, and on some nonpenetrants. They are called radial faults by many salt dome geologists. Radial faults subdivide the major fault blocks between the major faults, and are often important in controlling petroleum accumulations.
FAULT PATTERN CLASSIFICATION
Despite the importance of salt dome faulting, literature on the subject is limited. Wallace (1944) recognized two fault patterns, 1) "offset" patterns, both simple (with only one fault) and complex (with two faults downthrown in the same direction), and 2) "dome-with-graben" patterns, associated with "deep-seated" salt domes (top of salt below 6000 feet). Murray (1961, 1966) expanded upon Wallace, adding "compound off-set", "multiple offset", "radial" and "horst" fault patterns. Halbouty (1967) reviewed Wallace and presented an expanded classification similar to Murray's. The Murray and Halbouty references contain numerous examples of salt dome faulting.
Attempts by this writer to use the Murray and Halbouty classifications were not completely satisfactory. A majority of salt domes analyzed were in the same compound offset class. The fault pattern classification described below and illustrated in Figure 4 was developed by the writer and evolved out of the earlier classifications. It was used successfully in a study of faulting on 202 salt domes in the Coastal Salt Basin. Though appearing complex and unwieldy, the classification is actually flexible and easily applied. The various fault pattern names are purely descriptive. Patterns of major faulting are determined at objective section levels within the salt dome uplift area. Some faults are of regional extent, while others are restricted to just the uplift area. The smaller growth faults subdividing major fault blocks may have formed contemporaneously with the major faults, or later when strata were stretched laterally above the diapir or offset by continuing differential flank subsidence. These structural complications are easily accomodated in the fault pattern classification described.
The basic or single phase fault patterns are most common and involve one or more major growth faults in offset, compensated or crossed offset patterns, and are shown (Figure 4). Most of these major faults are ancestral, and were initially formed in the upper slope environment. The less important
radial fault pattern occurs on about 8% of the penetrant salt domes where no major fault is present. Only penetrants have radial fault patterns. The offset-radial pattern occurs occasionally on all types of salt domes, usually in combinations with additional faults.
Multiple phase patterns are superimposed combinations of two single phase patterns. They occur on about 35 percent of the salt domes in the sample, as offset plus compensated and crossed plus compensated patterns. As noted, multiple phase patterns may result from extensional stretching, differential flank subsidence, or both. This type of deformation appears to accelerate late in the continental shelf depositional phase and continue during the onset of denser alluvial sedimentation.
SALT DOME FAULTING STUDY
Faulting on 202 Coastal Salt Basin proven and possible salt domes was studied. Most structures are located in the Oligocene and Miocene producing trends, but some in the Eocene, Pliocene and midshelf Pleistocene trends are included (Figure 5). One hundred-sixty-nine are in Louisiana and 33 in Texas. Seventy produce mainly from the Oligocene or Eocene while 132 are principally Miocene or Pliocene producers. These 202 proved, probable or possible salt domes are classified as:
40 Penetrant Salt Domes (PSDs),
26 Semipenetrant Salt domes (SPSDs), and
136 Nonpenetrant Domes (NPDs).
The latter type is subdivided into two subtypes:
68 Proven/Probable Nonpenetrant Salt Domes (NPSDs) and
68 Possible Nonpenetrant Salt and/or Clay Domes (NPSCDs).
On the proven/probable nonpenetrants, either salt has been drilled below the objective section base or evidence of nearby salt, though undrilled, is strong. The possible nonpenetrant salt and/or clay domes are probable diapiric structures in which the
presence of salt is less certain. They usually have weaker gravity minima than where salt is proven. Their diapirs are probably composed of salt plus a large amount of associated diapiric clay or, possibly, a large amount of low density diapiric clay and no salt. Also included in the possibles are some structures which would probably be classed as proven/probable if the writer had gravity data available on them.
In the study area, onshore and out to about mid-shelf, most of the salt diapirs sampled are probably sourced from the "mother salt layer." Laterally displaced salt tongues are known to exist in this area, however, and some of the nonpenetrant salt masses sampled may be sourced from salt tongues. Farther south, under the outer shelf and upper slope, laterally displaced salt tongues and salt sheets become increasingly common and important (Brooks, 1989). Most of the Figure 1 diapiric ridges are probably sourced from salt sheets. Large volumes of shallow salt occur in the outer shelf to upper slope area, where extremely rapid sedimentation has occurred. The conclusions reached in this paper regarding salt dome faulting have not been tested in this area and may not be applicable there.
The field studies used were drawn mainly from the New Orleans, Lafayette and Houston Geological Societies oil and gas field volumes. Other sources include the GCAGS Transactions, AAPG Memoirs 9 and 14, the AAPG Bulletin, Bureau of Economic Geology and Louisiana Geological Survey publications, the Louisiana Conservation Commission Hearing Books and, occasionally, the writer's files. Space does not permit discussion or listing of the individual field studies and fault analyses. These data will be supplied at cost by the writer to interested parties upon written request.
Study Data Compilation And Presentation--The 202 structures were analyzed for major fault type, orientation and pattern by salt dome type. All major fault throws are not equal, however. A penetrant salt dome may have only one major fault of 2000 foot throw at objective levels, together with many smaller radial faults that have only a few hundreds of feet of throw. A small nonpenetrant dome may have three major faults, each with a throw less than 200 feet, plus some smaller faults. Yet, major faults are easily recognized regardless of throw or diapir class. Major faults divide a structure into major fault blocks with differing but characteristic stratigraphic, hydrodynamic and accumulation qualities. A major fault block may have considerable, little or no additional faulting. Data were tabulated by state and by producing trend, as Oligocene-Eocene or Miocene-Pliocene, and then combined by salt dome type of the entire Coastal Salt Basin as shown in Table 1.
Comparisons of major fault type, number, combinations and orientation by salt dome type are graphically illustrated in Figure 6, and similar comparisons of fault pattern data in Figure 7. The most common fault patterns on penetrant and semipenetrant domes and on the nonpenetrant subtypes are illustrated in Figures 8 and 9 respectively.
Fault Type And Pattern Variation--Fault characteristics of the 202 domes studied, vary significantly. Penetrants and semipenetrants are usually more heavily faulted and have more complex patterns. Penetrants and semipenetrants share some fault characteristics different from those of nonpenetrants, but important differences exist between them. More subtle but possibly significant differences occur between the nonpenetrant subtypes. The salt dome type samples studied are sufficiently large to be representative. In onshore south Louisiana
Table 1. Continued
they are in proportion to the actual dome distribution. The conclusions are believed to be relevent for that area. A larger Texas sample would be necessary for conclusions to apply equally to the Houston Embayment. Some of the more important conclusions are discussed below.
Louisiana-Texas Comparison--In general, the large 1000' + throw salt dome faults common in south Louisiana rarely occur in the Houston Embayment. Due to sample limitations, faulting comparisons can be made between the states of only Oligocene-Eocene producing trend structures. Little variation exists on the penetrants, excepting that generally more and smaller faults occur on the Texas domes; large single faults dominate in Louisiana. Both semipenetrant and nonpenetrant salt domes in Texas are more heavily faulted. Their patterns are more complex, with more crossed offsets, multiple phase compensation and counterbasinward oriented faulting than in south Louisiana. Domes of these types in the Texas Oligocene-Eocene trend are more similar to Miocene-Pliocene trend semi-penetrant and nonpenetrant salt domes in south Louisiana. No significant differences are recognized on the more subdued possible nonpenetrant salt and/or clay domes.
Producing Trend Comparison--Although Miocene-Pliocene domes are generally more heavily and complexly faulted than are Oligocene-Eocene domes, the differences in fault
charcteristics between the trends are subtle. Variations in fault type, orientation and pattern appear to be far more closely related to salt dome type than to the age of producing section.
Fault Type Comparison by Salt Dome Type, Shown in Figure 6--Numerous characteristic fault type and orientation trends and variations, related to salt dome type and of varying significance, can be drawn from these graphs. Only the more important conclusions are noted:
Graph A--Number of Major Faults: Multiple fault systems are most common on semipenetrants (SPSDs) and nonpenetrants (NPDs); all semipenetrants have multiple fault systems.
Graph B--Fault Type, Single Fault Systems: Tangential toward (Tgt) and tangential away (Tga) faulting increases regularly left to fight across the Buried Diapir Group (SPSD + NPSD + NPSCD). Transverse (Tr) faulting is dominant on penetrants (PSDs).
Graph C--Fault Type Combinations, Multiple Fault Systems: Tr-Tr combinations are common on penetrants and semi-penetrants, with Tgt-Tr combinations dominant on the latter. On nonpenetrants, Tgt-Tr-Tga and Tgt-Tr combinations are most common.
Graph D--Predominant Fault Type Expressed as "At Least One Present": A very important graph, where characteristic fault type associations with certain salt dome types are obvious.
Graphs E and F--Fault Orientation: Also very important. See the basinward and counterbasinward definition diagrams in Figure 3. "Basinward with counterbasinward present" means that most major faults are downthrown basinward, with minor faults and/or a smaller number of major faults down-thrown counterbasinward, etc. Counterbasinward faulting is dominant on penetrants and semipenetrants, and basinward faulting even more dominant on nonpenetrants. Basinward faulting increases and counterbasinward faulting decreases across the buried diapir group.
Summary: Salt dome type obviously has a strong influence on major fault type, number and orientation.
Fault Pattern Comparison by Salt Dome Type, Shown in Figure 7--Additional characteristic fault pattern trend and variation conclusions can be drawn from these graphs. The more important are noted;
Graph G--Occurrence of single phase fault systems: Multiple phase patterns are predominant only on semipenetrants.
Graphs H and I--Variation of single phase patterns: Except for crossed offset, most single phase patterns correlate closely to salt dome type. Compensated patterns are most common on semipenetrants, but decrease across the buried diapir group, where offset patterns increase with the increase in diapir burial depth. Grabens are common and horsts rare as compensated patterns. Penetrant salt domes, with their radial and offset radial patterns are somewhat "in a class by themselves."
Graph J--Variation of Multiple Phase Patterns: Multiple phase patterns are more common on semipenetrant and non-penetrant salt domes. Crossed offset compensated are dominant on semipenetrants. Offset compensated (graben) patterns are relatively common, with the grabens usually parallel to the major offset fault(s) on nonpenetrant salt domes, but perpendicular (or crossed) on penetrants and semipenetrants.
Graph K--Pattern Tendency by Salt Dome Type: A very important graph, with single and multiple phase patterns combined. All offset patterns (including offset compensated but excluding crossed offset) have been combined. All patterns involving large scale compensation have been combined. All crossed patterns of any type comprise a third category. Some structures are counted twice, as the purpose is to determine whether a tendency for offsetting, crossing or compensation is related to salt dome type. Except for penetrant salt domes and crossed patterns, tendency trends can be identified:
- offsetting increases and compensation decreases across the buried diapir group, as the depth of diapir burial beneath the objective section base increases.
- compensation (and crossing to some extent) are typical of semipenetrants.
- penetrants are a "different" salt dome type so far as faulting is concerned, with strong tendencies toward offsetting, above-average crossing and limited large scale compensation. Small scale compensation (Graph L) is more common on penetrants and semipenetrants.
Summary: Fault pattern characteristics are related to salt dome type and to the depth of burial, if any, of the diapir. Consistent linear trends are evident across the buried diapir group when the penetrant salt domes are blanked out in Figures 6 and 7. Considering the entire graphs, the unique position of the semipenetrants is evident. They are transitional with respect to the salt to objective section relationship, unique with respect to fault patterns and are the shallow end member of the buried diapir group. Counterbasinward faulting is characteristic of penetrants and semipenetrants, and basinward oriented faulting of the others. Subtle trends across the buried diapir group reflect nonpenetrant structural characteristics which become increasingly important on semipenetrants after diapirism ceases and burial begins. Examples of commonly occuring fault patterns are shown in Figures 8 and 9.
ORIGINS OF SALT DOME FAULTING VARIATIONS
Significant differences in fault type, orientation and pattern obviously exist among the salt dome types. What are the causes of these differences? Differences in faulting on nonpenetrants and on the penetrant and semipenetrant group are particularly
marked. But, semipenetrant patterns relate to those of non-penetrants, across the buried diapir group. Nonpenetrants and the penetrant/semipenetrant group existed in significantly different forms in the upper slope and outer shelf environments when objective section deposition began. Nonpenetrant diapirs were already inactive and buried in upper slope strata. Some formed low relief mounds or noses on the sea floor. Active growth faults crossed and flanked these buried diapir related uplifts, in basinward and counterbasinward orientations. Diapiric ridges, from which penetrants, semipenetrants and possibly some nonpenetrants developed, formed topographic highs. Tangential away peripheral growth faults with basinward and counterbasinward orientations flanked the diapiric ridges, downthrown into ridge flank depocenters.
The basic, ancestral fault diapir relationship for all diapiric structure types is that of tangential away, peripheral fault down the diapir face. The diapir forms the upthrown block, as shown in Figures 1 and 10. Multiple ancestral faults may flank a developing upper slope structure in both basinward and counterbasinward
orientations. As continental shelf deposition begins, usually one fault becomes dominant, with the other(s) being buried, and remains active into the alluvial deposition phase. Although an ancestral fault is tangential away with respect to the diapir, it may be tangential toward, transverse or tangential away with respect to the diapiric structure formed above or around it (Figure 10). The fault type variation is related to the distance between the diapir crest and the objective section. Where the objective section and the diapir are close (partly or completely penetrant), transverse and tangential away faults usually affect the diapiric structure. As this distance increases (nonpenetrant), tangential toward faulting of the diapiric structure becomes dominant. Thickness of objective section is also an important consideration.
Fault complexity and intensity are greatest where the objective section and diapir are in close contact. They diminish as the distance increases between diapir crest and objective section. Penetrant and semipenetrant salt diapirs are exposed at or close to the depostional surface during deposition of all or part of the objective section. Differential subsidence occurs contemporaneously along the flanks due to salt withdrawal. Ancestral fault planes may be steepened at depth on the flanks of some penetrants. Faulting thus develops differently on penetrent and semipenetrants than it does on structures where simple draping and stretching of strata over a buried, nonpenetrant
diapir occurs. Only limited differential subsidence can occur along the flanks of these diapirs.
Large young faults are common on penetrants and semipenetrants. Formed in response to salt withdrawal and flank subsidence during objective section deposition, their orientation, type and pattern may be unrelated to the ancestral upper slope fault(s) which remain active while these younger faults form. With nonpenetrants, younger faults caused by lateral strata stretching above buried diapirs usually occur in patterns and orientations directly related to the ancestral upper slope fault(s).
Penetrants and semipenetrants have sufficient relief during upper slope deposition to strongly influence arriving sediment distribution. One or two flanks tend to receive thicker, sandier sediments. Usually the north, east or west flanks subside more rapidly and localize subsequent deposition. The counterbasinward ancestral major faults characteristic of penetrants and semipenetrants are downthrown toward these depocenters, and may be formed in this manner. They are usually transverse or tangential away with respect to the objective section on penetrants, and transverse or tangential toward on semipenetrants (Figure 10). Where objective section piercement continues (penetrants), these upper slope faults remain dominant even as younger faults form. If the diapir is buried within the objective section (semipenetrants), younger faults may form in the same or a different pattern, with the ancestral upper slope fault(s) remaining active. The ancestral fault pattern is thus modified to varying degrees after the diapir is buried by younger, secondary faults in compensated or crossed patterns. The original ancestral pattern remains, with the secondary pattern imposed. The multiple phase fault patterns most common on semipenetrants are a result of this transition from salt piercement to salt burial during objective section deposition.
Nonpenetrants, buried in the preobjective section slope environment, inherit much simpler ancestral fault patterns. Their limited relief at the depositional surface may be sufficient to affect sediment distribution. Strata deposited above the buried diapir are usually thinner than around the flanks. They are often thickest downthrown to basinward oriented, diapir peripheral growth faults on the basinward side of the structure. Offset patterns predominate, but may be modified by younger compensating movements. Multiple phase patterns result, in response to stretching and adjustment of objective strata. Fault pattern complexity and compensation are greater on nonpenetrant salt domes than on salt and/or clay or clay domes. On the latter, the diapir often appears to be farther removed from the objective section, perhaps reflecting the greater mobility of salt compared to diapiric clay, or a relatively smaller or deeper salt mass.
Although a salt diapir on the upper slope may be flanked by tangential away, peripheral fault systems with basinward and counterbasinward orientations, only one system usually survives after continental shelf deposition begins. Counterbasinward oriented systems survive and become increasingly complex on penetrants and semipenetrants. Single or multiple offset, basinward oriented systems almost always survive on nonpenetrants. Nonpenetrants with relatively close salt involvement may also have secondary compensated patterns imposed.
Exploration around heavily drilled salt dome structures usually involves testing previously unrecognized fault traps or depocenters. Analysis of the fault characteristics of salt domes may make it easier to identify the ancestral, upper slope faults which play an important role in this type of exploration. Extending the ancestral fault pattern downward to the base of objective section may help:
- define the original upper slope shape and limits of diapiric uplifts,
- separate prospective deep flank depocenters from non-prospective diapiric mass areas.
- identify previously unrecognized prospective fault traps.
- offer clues to the existence of previously unsuspected salt overhangs on penetrants.
- better delineate at depth the limits of abnormally pressured diapiric clay sheath around all types of salt diapirs.
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