Hydrocarbon migration may have three results:(1)accumulation

(2)diffusion in rocks and fluids

or (3) loss due to interactions with fluids and surrounding rocks or surface exposure.The latter is defined here as dissipation.The dissipation is most active during hydrocarbon migration and is closely linked to hydrocarbon accumulation

thus study of the phenomenon become an indispensible part of petroleum geology research

and petroleum exploration and resource assessment.Hydrocarbon dissipations can be divided into four types according to the differences of quantity and quality of hydrocarbons before and after dissipation:quantity decrease

phase transformation

composition alteration

and quality conversion

reflectting the differences in environment

conditions and characteristics of dissipation.The quality conversion often occurs at surface or near surface

where hydrocarbons will interact with surrounding rocks

pore fluids or air.The epigenetic products and minerals resulted from these interactions generally have no apparent features of hydrocarbons

but the amount of dissipated hydrocarbons is huge

thus much attention should be given to this phenomenon.These products in combination with direct evidences of hydrocarbon dissipation ( such as seepage

bitumen and heavy oil) can not only serve as an important means to address the puzzle of hydrocarbon dissipation and its scale

but also provide effective clues and significant indicators for tracing hydrocarbon pathways

searching for oil and gas pools

and evaluating the scale of dissipations.This paper analyzes and discusses the types

basic characteristics

geochemical properties

environments of formation and distributions of various epigenetic products related with gas dissipation in the Upper Paleozoic in northeastern Ordos Basin.