Characteristics and distribution of tertiary algal limestones reservoirs in Qaidam basin

摘要

Tertiary algal limestones reservoirs in Qaidam basin are characterized by wide distribution and being the most important in carbonates reservoirs. Tertiary algal limestones reservoirs in western Qaidam basin have been found in YUEXI, YUEDONG, SHIZIGOU, HUATUGOU and NANYISHAN regions etal, and distributed from E_3~2 to N_2~2. Algal limestones reservoirs in upper member of XIAGANCHAIGOU formation(E_3~2) have been found in YUEXI and YUEDONG regions, those in SHANGGANCHAIGOU formation(N_1) have been found in SHIZIGOU and HUATUGOU regions, those in XIAYOUSHASHAN formation(N_2~1) and SHANGYOUSHASHAN formation(N_2~2) have been found in NANYISHAN region. Algal limestones reservoirs in E_3~2 and N_1distributed mainly to west (southwest) region of YINGXIONGLING depression, those in N_2~1 and N_2~2 distributed mainly to east(northeast) region of YINGXIONGLING depression. Distribution of algal limestones in Qaidam basin is mainly controlled by lake level changes. The types of tertiary algal limestones in Qaidam basin are mainly composed of algal masses limestone, algal laminar limestone, algal stromatolithic limestone, algal micrite limestone and algal oosparite. Algal limestones reservoirs spaces are predominant with solution pores, solution fissures and mold pores. They are possibly controlled by atmospheric water solution and subsurface organic acid solution. Algal masses limestone, algal laminar limestone and algal stromatolithic limestone are favorable reservoirs, algal micrite limestone and algal oosparite are the secondary. Algal limestones reservoirs in YUEXI and YUEDONG regions are the best,average porosity is 20-30%, average permeability is near 100md.Algal limestones reservoirs in NANYISHAN region are the secondary, average porosity is 13-18%, average permeability is 1-10md.Algal limestones reservoirs in HUATUGOU and SHIZIGOU regions are the worst, average porosity is 8-15%,average permeability is 1-10md.