Metamorphic textures and mineralogy develop progressively over several hundreds of feet/meters of drilling. Without careful examination by the Geologist, and the recognition of the subtle changes in mineralogy and texture, much time and money can be wasted by drilling past the economic basement.
Since the change from sedimentary to metamorphic rocks is transitional, even the most experienced geologist will require time and footage to recognize and confirm the event. Experience shows that the majority of footage cut of metamorphic rocks is of low metamorphic grade which was not recognized, simply because the geologists at the wellsite did not consider the possibility.
The texture of a metamorphic rock is a unique product of its mineralogy and metamorphic conditions. Even if a complex assemblage is not identifiable at the wellsite, a combination of minerals and textures should allow the rock to be characterized.
Grains are platy to elongate and oriented parallel or subparallel; foliated if fabric is planar or lineated if the fabric is linear; micaceous and tabular minerals are common and usually well enough developed to be visible.
Common rock types are:
- Schist - grains can be seen without using a microscope
- Phyllite - all grains of the ground-mass are microscopic, but cleavage surfaces have a sheen caused by reflection of platy or linear minerals. Commonly corrugated.
- Slate - grains are microscopic, very cleavable, usually tougher than shale.
Grains are approximately equi-dimensional, platy and linear grains are randomly oriented. No foliation is developed. Common rock types are based on compositional type (i.e. Quartzite, Marble).
Grains are irregular and generally interlocking, and microscopic. They exhibit a hackly fracture, are very hard and fresh surfaces show a sugary coating which will not rub off. Rocks of this type are called “Hornfels”.
Platy or linear grains subparallel, but so subordinate or so unevenly distributed that the rock has only a crude foliation. Especially common in metamorphosed granular rocks. Common rock types are “Gneisses”.
Clastic textures resulting from breaking and grinding with little if any recrystallization. Characterized by angular, lensoid, or rounded fragments in a fine-grained and commonly streaked or layered ground-mass. The ground-mass is usually rock flour. Common rock type is “Mylonite”.