四川盆地元坝气田长兴组古油藏的定量恢复及油源分析
Paleo-Oil-Reservoirs Reconstruction and Oil Correlation of Changxing Formation in the Yuanba Gas Field, Sichuan Basin

目前虽认为元坝气田长兴组天然气主要为原油裂解气，但尚未进行该气田的古油藏的定量恢复并计算原油裂解气的资源量，并系统分析古油藏原油的来源.长兴组储层普遍可见固体沥青，是原油裂解的直接产物，且原油裂解在本区构造抬升变形前就已经完成，因此储层沥青可用来识别古油层的分布.根据储层沥青的纵向和平面分布，确定了7个可靠的古油藏和4个可能的古油藏，并运用容积法恢复出本区聚集的古原油为6.14×108t，计算出相应的原油裂解气为3807.08×108m3，远大于现今气田的天然气探明储量，表明原油裂解气可以提供充足的气源，进一步证明了天然气主要为原油裂解气.通过长兴组储层沥青与不同层系烃源岩干酪根的碳同位素δ13C值对比，并结合烃源层分布和TOC等资料，确定古油藏原油主要来源于有机质类型以II型为主的上二叠统吴家坪组烃源岩，其次为长兴组/大隆组烃源岩.后者主要分布在广元-南江-通江地区，该区的天然气勘探不能忽视该套烃源岩的生烃潜力与成藏贡献.
The natural gases from the Changxing Formation (P2ch) in the Yuanba gas field are mainly derived from oil cracking, but the volume of oil cracking gas has not been evaluated quantitatively and the source of the paleo oil has not been studied systematically. Solid bitumen, commonly developed in the P2ch reservoirs, is a direct product of oil cracking which had finished before the regional uplift and structure deformation in the Yuanba gas field. As a result, the paleo oil zone can be recognized by the solid bitumen. Basing on the lateral and vertical distribution of solid bitumen, seven reliable and four probable paleo oil reservoirs have been found out. The paleo oil accumulated in these reservoirs by volumetric method is about 6.14×108t, and the corresponding oil cracking gas is about 3807.08×108m3. The volume of oil cracking gas is much larger than the natural gas found in the Yuanba gas field, which indicates that the oil cracking gas can supply sufficient gases for the present gas reservoirs, and further proves that the natural gases are mainly derived from oil cracking. The δ13C correlation of solid bitumen and kerogens from different source rocks, and total organic carbon (TOC) data show that the paleo oil was derived from type Ⅱ source rock mainly in the Upper Permian Wujiaping Formation, and then in the Changxing/Dalong Formation. The latter is mainly developed in Guangyuan Nanjiang Tongjiang area, and the hydrocarbon generated by this source rock cannot be neglected during the natural gas exploration in this area