FORMATION AND DISTRIBUTION OF BIOGAS RESERVOIR IN THE LULIANG BASIN

Biogas reservoir in the Luliang basin is mainly vertically distributed in the 3rd and 4th members of Ciying Formation in the Lower Tertiary,with a depth ranging from 400 to 800 m.The major factors which control biogas reservoir include: 1)The Luliang basin pulled apart and subsided dramatically in the Tertiary and sediments compensated rapidly.Sedimentation rate of the Ciying Formation reached 200 to 500 m/Ma,forming favorable structural setting to biogas reservoir;2)The 3rd and 4th members of Ciying Formation developed humic organic matter of rich herbage,moreover,the 2rd member of Ciying Formation mainly developed humosapropelinite;3)Medium burial depth of the 3rd and 4th members of Ciying Formation in deep depression,and the geothermal temperature of 30~60℃ are favorable to the increase of methane bacteria;4)Nitrate and sulphate were nearly depleted below 400 m,therefore methane bacteria increased dramatically;5)In this depth range,pore volume of mudstone reaches 10~140 μm,which motivated dramatic activity of methane bacteria;6)In this depth range,methane formed rapidly in mudstone to prevent the diffusing of underlying methane reservoir.Distribution of biogas reservoir laterally is controlled by sedimentary facies and structural elements.Favorable sedimentary facies include transition zone between deep depressions of the 3rd and 4th members of Ciying Formation which are major gas producing layer,and all kinds of clastic rocks.Favorable structural elements include deep depression and slope intrazone.The overlapped zone is favorable to biogas reservoir formation:1)The 3rd and 4th members of Ciying Formation have big thickness and medium burial depth.They are in the period of dramatic activity of methane bacteria and have big gas producting ability;2)Seal thichness is big with good sealling condition;3)Fault density is far lower than that of border faulted zone and slope outer zone,which is favorable to the preservation of biogas reservoir;4)Sand-shale ratio is mediate and lithologic and lithologic-structural trap are developed;5)There is a barrier between reservoir and atmospheric water of basin margin forming reducing environment with low sulphate which is favorable to methane bacteria activity.