南堡凹陷奥陶系碳酸盐岩岩溶残丘圈闭勘探潜力及意义

Exploration potential and significance of Ordovician carbonate karst monadnock traps in Nanpu sag

通过岩心描述、露头观察、成像测井解释、试油资料统计及地震剖面解释,结合区域地质资料分析,借鉴鄂尔多斯盆地、塔里木盆地及渤海湾盆地下古生界碳酸盐岩潜山岩溶储层形成及成藏模式,深入研究了南堡凹陷奥陶系碳酸盐岩潜山岩溶储层的特征及主控因素、发育模式,提出南堡凹陷奥陶系潜山岩溶残丘圈闭是该区下古生界潜山油气勘探的潜在新领域.南堡凹陷奥陶系潜山岩溶储层主要是缝洞型储层,垂向上主要分布于水平潜流带及垂直渗流带,有利储层主要分布于潜山不整合面之下＜100m的范围内、少数也可深达200m以上.该储层的形成主要受控于喜马拉雅期暴露大气淡水溶蚀作用,当时的岩溶古地貌及断裂-裂缝的发育控制了奥陶系潜山岩溶储层的发育与分布,南堡凹陷中南部总体处于岩溶斜坡并形成了优质的潜流带缝洞储层,仅其中的次级高地因发育风化残积层及垂直渗流带较厚等原因导致储层变差.亮甲山组顶部及下马家沟组顶部分别发育1套厚达20～30m的膏溶角砾岩,因其泥质含量高可作为油气盖层/隔层,与上覆古近系 白垩系区域盖层及下马家沟组、上马家沟组2套潜山岩溶储层形成2套有利储盖组合,并在南堡凹陷中南部斜坡区形成多个岩溶残丘地层-成岩岩性圈闭,成为南堡凹陷下古生界潜山有利油气勘探新领域.类似潜山岩溶残丘圈闭在渤海湾盆地其他地区如冀中坳陷、黄骅坳陷大港油田探区及济阳坳陷也有分布,可作为渤海湾盆地下古生界潜山油气勘探的新领域,值得深入研究与探索.

Based on core description, outcrop observation, imaging logging interpretation, oil testing data statistics and seismic profile interpretation in combination with regional geological data analysis, as well as the formation and accumulation modes of Lower Paleo- zoic carbonate buried-hill karst reservoirs in Ordos Basin, Tarim Basin and Bohai Bay Basin, a deep study was carried out on the characteristics, main controlling factors and development modes of Ordovician carbonate buried-hill karst reservoirs in Nanpu sag. It was put forward that the Ordovician buried-hill karst monadnock traps in Nanpu sag were a new potential field for Lower Palaeozoic bur- ied-hill hydrocarbon exploration in this region. The Ordovician buried-hill karst reservoirs in Nanpu sag are dominated by fracture-cave reservoirs, mainly distributed in horizontal phreatic zone and vertical vadose zone in the vertical profile. Favorable reservoirs are primari ly distributed beneath the buried-hill unconformity surface within the depth of 100 m, while a few can reach the depth of more than 200 m. The reservoir formation was mainly controlled by Himalayan exposed atmospheric fresh water dissolution. In the meantime, the de- velopment and distribution of Ordovician buried-hill karst reservoirs was controlled by the development of karst paleo-geomorphology and fault-fractures. The south-central Nanpu sag is generally located in karst slopes and form high-quality phreatic-zone fracture-cave reservoirs, while the reservoirs become poor in secondary highlands due to the development of weathering eluvium and thick vertical va- dose zones, etc. A set of 20-30-m-thick gypsum breccias are developed at the top of Liangjiashan Formation and Lower Majiagou For- mation respectively, which can be taken as hydrocarbon caps/barriers because of high shale content. In combination with the overlying Paleogene-Cretaceous regional caps and two sets of buried-hill karst reservoirs in Lower and Upper Majiagou Formation, two sets of fa-vorable reservoir-cap assemblages were formed, as well as multiple karst monadnock reservoirs and diagenetic lithologic traps in the cen- tral and southern slopes of Nanpu sag, thus forming a new field for favorable Lower Palaeozoic buried-hill hydrocarbon exploration in Nanpu sag. Similar buried-hill karst monadnock traps are also distributed in other areas of Bohai Bay Basin, such as Central Hebei De- pression, Dagang oilfield exploration area of Huanghua Depression and Jiyang Depression, which can be used as a new field for Lower Palaeozoic buried-hill hydrocarbon exploration in Bohai Bay Basin, which is worthy of further research and exploration.