mining or spent oil shale disposal aboveground as spent oil shale stays underground. By heating method: The method of transferring heat from combustion products to the oil shale may be classified as direct or indirect. While methods that allow combustion products to contact the oil shale within the retort are classified as direct, methods that burn materials external to the retort to heat another material that contacts the oil shale are described as indirect By heat carrier: Based on the material used to deliver heat energy to the oil shale, processing technologies have been classified into gas heat carrier, solid heat carrier, wall conduction, reactive fluid, and volumetric heating methods. Heat carrier methods can be sub-classified as direct or indirect. The following table shows extraction technologies classified by heating method, heat carrier and location (in situ or ex situ). By raw oil shale particle size: The various ex situ processing technologies may be differentiated by the size of the oil shale particles that are fed into the retorts. As a rule, gas heat carrier technologies process oil shale lumps varying in diameter from , while solid heat carrier and wall conduction technologies process fines which are particles less than in diameter. By retort orientation: "Ex-situ" technologies are sometimes classified as vertical or horizontal. Vertical retorts are usually shaft kilns where a bed of shale moves from top to bottom by gravity. Horizontal retorts are usually horizontal rotating drums or screws where shale moves from one end to the other. As a general rule, vertical retorts process lumps using a gas heat carrier, while horizontal retorts process fines using solid heat carrier. By complexity of technology: In situ technologies are usually classified either as true in situ processes or modified in situ processes. True in situ processes do not involve mining or crushing the oil shale. Modified in situ processes involve drilling and fracturing the target oil shale deposit to create voids in the deposit. The voids enable a better flow of gases and fluids through the deposit, thereby increasing the volume and quality of the shale oil produced. Ex situ technologies Internal combustion Internal combustion technologies burn materials (typically char and oil shale gas) within a vertical shaft retort to supply heat for pyrolysis. Typically raw oil shale particles between and in size are fed into the top of the retort and are heated by the rising hot gases, which