Strike Fault
A strike fault (also called a strike-slip fault in general structural geology usage, or specifically a fault whose strike is parallel to the regional structural trend or to the strike of the bedding) is a fault that runs parallel to the strike of the rock layers it cuts through, so that the fault plane and the bedded rock it intersects trend in the same compass direction; in classical structural terminology, strike faults may be either normal (downthrown on the downdip side) or reverse (upthrown on the updip side) depending on the sense of displacement relative to bedding dip, and the parallel orientation of the fault to bedding can make them difficult to distinguish from bedding planes in areas of poor exposure or limited well control; in petroleum geology, strike faults are significant because their orientation parallel to bedding and regional structure commonly results in their juxtaposing reservoir-quality rock against itself or against different facies of the same formation, which can either create lateral seals where the fault core is impermeable or create cross-fault communication where the fault is open and transmissive; the term is sometimes applied loosely to any fault with a strong sub-parallel relationship to the dominant structural grain of the basin, including thrust faults in fold-and-thrust belts (which are also often sub-parallel to the bedding they cut), distinguishing them from cross-faults or transverse faults that cut obliquely or perpendicularly across the structural trend.
Key Takeaways
- The geometric relationship between fault strike and bedding strike determines the map pattern of a strike fault and its effect on reservoir connectivity: in a strike fault that perfectly parallels the bedding strike, the fault trace on a map runs parallel to the outcrop pattern of the bedded rock, and in the subsurface the fault appears on seismic sections as a sub-vertical plane adjacent to the bedding reflectors with no apparent offset in the reflector dip direction (making the fault difficult to pick on seismic in the dip direction and more visible on strike-oriented lines where any dip-slip component of the fault displacement creates a reflector offset); in plan view (map view), strike faults appear as long linear features parallel to the structural contours of the geological map, following the trend of ridges, valleys, or fold axes that reflect the underlying structural grain; in fold and thrust belt settings, strike faults are commonly thrust faults (reverse dip-slip) that parallel the fold limbs, making them challenging to distinguish from the adjacent bedding by both seismic reflection and surface mapping.
- Sealing behavior of strike faults is critical for petroleum trap integrity in thrust belt and fold-and-thrust belt settings: a strike fault that juxtaposes reservoir rock against the same or lower-quality reservoir rock may be sealing (if the fault gouge developed in the fault core during displacement is fine-grained and clay-rich, reducing permeability across the fault) or transmissive (if the fault zone contains dilational fractures or if the clay smear in the fault core has been eroded or breached by subsequent deformation); the juxtaposition of reservoir against reservoir in a strike fault (common when the fault is sub-parallel to bedding and displaces different facies of the same formation laterally rather than vertically) requires analysis of the fault seal properties rather than juxtaposition seal (the latter being the dominant seal type for faults with large dip-slip components that place reservoir against cap rock); shale gouge ratio (SGR) and clay smear potential (CSP) calculations, applied to the column of rock moved past the fault surface during displacement, provide quantitative estimates of the clay content of the fault gouge that determine whether the strike fault acts as a barrier or conduit for hydrocarbon migration and production.
- In foreland fold-and-thrust belt settings, the distinction between strike faults and the thrust faults that define the structural style requires careful analysis because both are typically sub-parallel to the regional structural trend: thrust faults in fold-and-thrust belts dip gently in the direction of tectonic transport (toward the foreland, commonly at 20-40 degrees from horizontal), while strike faults in the same setting may be either steeply dipping reverse faults on the steep back limbs of fault-propagation folds or tear faults (transfer faults with strike-slip displacement that accommodate differential displacement along the thrust front); tear faults are a specific type of strike fault that cut through the thrust sheets perpendicular to or oblique to the thrusting direction, accommodating the lateral variation in thrust fault displacement; the intersections of tear faults with thrust faults create complex fault network geometries that both compartmentalize reservoirs (limiting cross-fault drainage) and create structural closures at the fault intersections where juxtaposition of differently pressured reservoir compartments concentrates hydrocarbons.
- Cross-formational communication along strike faults is a subsurface fluid dynamics concern when the strike fault connects formations at different burial depths and pressures: in a steeply dipping normal or reverse strike fault that cuts through a sedimentary section, the fault zone may provide a permeable pathway connecting shallow (low-pressure) formations with deep (high-pressure) formations, allowing upward fluid migration along the fault plane; in a petroleum context, this upward leakage is the primary risk to structural trap integrity where a strike fault cuts the cap rock of the trap, providing a migration pathway for hydrocarbons that have accumulated in the trap to escape to shallower, unconfined levels; the assessment of cross-formational communication along strike faults uses fluid inclusion analysis of fault-plane mineral cements (which record the temperatures and pressures of past fluid migration events), isotopic characterization of fault-zone water samples compared with formation water in adjacent formations, and petroleum geochemical fingerprinting of shows in formations that have no direct reservoir connection other than through the fault zone.
- Petroleum exploration strategy in strike fault-dominated basins must account for the structural style imposed by the orientation of major strike faults relative to the regional stress field: in transtensional basins controlled by strike-slip fault systems (such as the California basins associated with the San Andreas system), the dominant structural traps are flower structures, pull-apart basins, and fault-bend folds adjacent to the major strike faults; the strike faults serve simultaneously as migration pathways (where open, transmissive fault zones allow hydrocarbons to migrate from deeper source kitchens into shallower traps along the fault) and as seals (where compressive segments of the fault zone are tightly closed and impermeable); in compressional settings with strike fault-dominated deformation such as the Zagros fold-and-thrust belt, the strike faults define the boundaries of individual structural panels with potentially different reservoir pressures, fluid contacts, and hydrocarbon compositions on either side, requiring independent evaluation of each fault-bounded compartment rather than a single-compartment reservoir model.
Fast Facts
The structural terminology of strike faults, dip faults, and oblique faults was formalized in 19th-century geological mapping as practitioners needed vocabulary to describe the orientation of faults relative to the bedding they cut. The terms derive from the fundamental geological measurements of strike (the compass direction of a horizontal line on a tilted rock surface) and dip (the angle of maximum inclination of the tilted surface from horizontal). In modern structural geology, the terminology has largely been superseded by kinematic descriptors (normal fault, reverse fault, strike-slip fault) that describe the sense of relative motion rather than the geometric relationship to bedding. The term "strike fault" is retained primarily in older petroleum geology literature and in fold-and-thrust belt contexts where the parallel relationship between faults and bedding is the defining structural characteristic of the geological setting.
What Is a Strike Fault?
A strike fault runs in the same direction as the rock it cuts. In a tilted sedimentary section, the beds dip in one direction and their upper edges strike in the perpendicular direction; a fault that parallels those upper edges is a strike fault. Because the fault runs along rather than across the bedded structure, it tends to juxtapose the same or similar rock types across the fault plane, making it geometrically different from a dip fault that cuts across multiple formations and can create dramatically different juxtapositions. In a fold-and-thrust belt, the dominant faults are thrust faults that parallel the fold axial traces and the bedding they have displaced: classic strike faults in the structural sense. In a wrench or transpressional basin, strike-slip faults with horizontal motion parallel to the bedding strike create the same geometric relationship. In both cases, the orientation parallel to the structural grain shapes how the basin's petroleum system works: how migration occurs along the fault, how compartmentalization between fault-bounded blocks is organized, and whether adjacent wellbores are in communication with each other or are isolated by fault seals that happen to parallel the reservoir they bound.
Synonyms and Related Terminology
Strike fault is related to but not identical with strike-slip fault: a strike-slip fault has predominantly horizontal motion parallel to the fault strike, while a strike fault may have any sense of displacement (normal, reverse, or strike-slip) as long as its orientation is parallel to the bedding strike. Related terms include dip fault (a fault whose strike is perpendicular to the regional bedding strike, cutting across the structural grain and typically juxtaposing different formations or significantly different facies of the same formation on either side, in contrast to the strike fault which runs parallel to the bedding strike), tear fault (a specific type of strike fault in thrust belt settings that has predominantly strike-slip displacement, accommodating lateral variation in thrust displacement along the fold-and-thrust belt front, creating compartment boundaries between structural panels with potentially different displacement histories), fault seal (the capacity of a fault zone to prevent cross-fault fluid flow, determined by the clay content of the fault gouge (from shale gouge ratio calculations), the diagenetic cements in the fault zone, and the capillary entry pressure of the fault core relative to the buoyancy pressure of any hydrocarbon column it is asked to contain), thrust fault (a low-angle reverse fault in which the hanging wall has moved up and over the footwall, typically striking parallel to the regional structure in fold-and-thrust belts and therefore constituting a major class of strike faults in compressional tectonic settings), and structural grain (the dominant orientation of faults, folds, and structural features in a geological province, determined by the regional stress field and tectonic history, and the reference direction to which strike faults are defined as parallel and dip faults are defined as perpendicular or oblique).
Why Strike Fault Orientation Relative to Drainage and Migration Pathways Controls Trap Integrity
A fault that parallels the structural trend is a fault that parallels the updip migration direction. Hydrocarbons migrating from deep source kitchens to shallower traps tend to follow the dip of carrier beds in the updip direction, which is perpendicular to the structural strike. A strike fault that cuts across this migration path at an angle is a lateral barrier: it forces migrating hydrocarbons to either find a gap in the fault or pool against the fault plane until buoyancy pressure exceeds the fault's capillary entry pressure and they leak through. Where the fault seals, the updip pooling creates a lateral trap that concentrates hydrocarbons without requiring a four-way structural closure. Where the fault is transmissive, the migration continues through the fault plane and the apparent structural closure the fault creates is leaking. Distinguishing sealing from non-sealing strike faults before drilling the prospect is the geological and geochemical analysis that determines whether the exploration concept has trap integrity. Getting that analysis right in a thrust belt or wrench basin environment where strike faults are ubiquitous separates productive exploration programs from those that drill good structural closures that turn out to have no hydrocarbon columns because the faults they depend on for lateral sealing are open to the surface.