Faults, Fractures, and Structure

Extension, compression, strike-slip, salt, gravity: the style names the stress regime, and Anderson's theory says why each regime makes the faults it makes. Classify the style first and every later pick has a context.

Throw, heave, slip, and dip are one right triangle, and the shale gouge ratio is the number that turns a measured fault into a seal-risk statement. This is the quantitative core of structural interpretation.

A fold's shape betrays its mechanism, drape, fault-bend, fault-propagation, or detachment, and salt writes its own structural grammar of diapirs, rim synclines, and minibasins. Both are trap makers and both are misread routinely.

Horizon and fault picks become a 3D framework, the framework becomes a structure map, and the map's closures and spill points become a ranked prospect. This is where structural interpretation earns its keep.

The image log is the unrolled borehole wall, azimuth by depth, and it is the only measurement that sees a fracture directly. A dipping plane crosses it as a sine wave, and that one fact unlocks the whole chapter.

The sinusoid's height gives the dip and its low point gives the azimuth; the tadpole plot stacks those measurements into the green, red, and blue motifs that distinguish structure from channels and foresets.

An observed dip is a structural tilt plus a depositional dip. Remove the structural vector and the bedding goes flat, and the residual foreset dips point down the paleocurrent: two geologies separated by one subtraction.

Open fractures read dark and flow, sealed ones read bright and block, and the drilling-induced pairs and breakouts point at SHmax. One log, two stories: the rock's fractures and the field's stress.

A crack is a compliant flaw with a direction: soft across its faces, stiff along them. Crack density becomes the normal and tangential weaknesses, and Hudson and linear slip map those to the Thomsen parameters of an equivalent HTI rock.

Fluid props the normal weakness but never the shear one, so azimuthal P anisotropy is fluid-sensitive while shear-wave splitting is fluid-blind and reports geometry alone. Ruger's AVAz ellipse and the split shear wave are the two detectors, and together they read both strike and fill.

Three geological choices, strike, crack density, and fluid, become a complete HTI velocity model: an isotropic background plus an oriented perturbation, the structure nearly all earth modeling shares.

The framework has exactly three building blocks: horizons bound the zones, layers subdivide them, and faults break the reservoir into blocks. Every property and every flow run will live on this skeleton.

Juxtaposition plus shale gouge decides whether the fault seals or leaks, and the transmissibility multiplier carries that verdict into the simulator. Sealing faults mean compartments, and compartments mean wells.

A fault that seals today can slip tomorrow: resolve the stress tensor onto the plane and the slip tendency, shear over effective normal stress, says how close it sits to the frictional limit. Orientation is destiny, and the stereonet shows which faults in the framework are one pressure change from moving.

Proportional, follow-top, and follow-base each encode a depositional assumption, so the layering scheme is a flow decision, and a box grid that ignores dip mixes rocks a corner-point grid keeps apart.

The framework is the one part of the model everything later inherits, so it is checked before any property is added. Crossing horizons mean negative thickness, negative volumes, and a crashed simulator; the capstone is catching that before anyone downstream does.