Mechanism for Emplacement of Piercement Diapirs

Abstract

The buoyancy model long has been regarded as the principal mechanism of diapir emplacement. This hypothesis, however, describes only nonpiercement diapirism; a different theory is required to explain piercement diapirism. By assuming that clastic overburdens are cohesive (e.g., rigid plastic) rather than viscous materials, we can recognize emplacement histories and mechanical processes significantly different from those determined from the buoyancy theory. If an overburden has strength, diapirism can be treated as a hydraulic process: the mobile substrate (salt or shale) is a fluid and the diapir is a pressurized hole. The theory presented herein describes how the pressurized fluid might deform the overburden under different boundary conditions. The most important result of this theorizing shows that the emplacement of each diapir--whether it is salt or shale--must be explained in terms of its depositional history as well as its mechanical processes. That is, progradation rate, sedimentation rate, lithology, etc., all combine to influence the number, size, shape, and hydrocarbon-trapping ability of diapirs. Consequently, attempts to make predictions may be best rewarded by examining burial history rather than such material properties as viscosity and shear strength. In general, we observe that slow sedimentation rates produce small, discrete diapirs with vertical sides, whereas rapid sedimentation rates produce larger, less discrete diapirs commonly having nonvertical sides. For our predictions, we attempt to show the conditions controlling the maximum number of diapirs in an area and whether they form traps for hydrocarbons. Specifically, diapir diameter may affect the number of diapirs, because diapirs having larger diameters may cause smaller, adjacent diapirs to cease growing. We use this relation to estimate maximum concentrations of diapirs. In addition, we can estimate the thickness of the pierceable overburden using a crude material-balance calculation which includes diapir volume, salt thickness, and radius of withdrawal. This is important in determining whether a diapir forms a trap for hydrocarbons. Piercement diapirism typically occurs by extrusion or alternates between extrusion and intrusion. Here we describe emplacement in three mechanically distinct fashions: extrusion, vertically intrusion, or vertically (forceful) intrusion. This departure from familiar terminology is to recognize two mechanically distinct types of intrusion. The terms constrained and unconstrained describe whether a diapir can move freely in reaction to any unbalanced stresses. Unconstrained intrusions can grow as sediments accumulate around and over the diapir. Constrained intrusions, however, cannot move freely; to move they first must fracture the overburden. This concept may explain why diapirs can stop growing even though diapiric material is abundant. End_Page 1561------------------------------ The depth to the top of an intrusion is important because it may determine whether a diapir is or unconstrained. Vertically diapirism is of two types: thin overburden results in vertical but not horizontal growth; thick overburden results in horizontal expansion before vertical growth. Hence overburden thickness is important because it determines both the direction and mechanism of diapir movement. This theory shows how salt diapirs may pierce thicker overburdens than shale diapirs. During burial, the ratio of salt to the overburden density increases, and the salt diapir may stay near the surface and remain vertically throughout sedimentation. In contrast, the ratio of shale density to its overburden may decrease during burial, thereby causing intrusion at a slower rate than sedimentation. This results in deeper burial of the shale diapir; the consequence is vertical constraint, and perhaps cessation of the diapirism.