西沙海域生物礁地球物理特征及油气勘探前景

Geophysical features and hydrocarbon exploration prospectin of eeef in Xisha offshore

生物礁是指由造礁生物营造的碳酸盐岩有机建造.生物礁油气藏具有储量大、产量高的特点,是全球重要油气藏类型,也是南海主要勘探对象之一.西沙海域位于南海北部,前人研究成果表明该区自中新世以来具有良好的成礁环境,生物礁大规模发育.本文在西沙海域地质背景进行分析的基础上,以地震资料为基础,以一个典型生物礁为例,综合分析了该区生物礁的地球物理响应特征.首先,生物礁具有典型的地震反射特征:丘状外形、顶界面为强振幅连续反射、底界面连续性变差、内部反射杂乱、两侧有上超、上覆地层披覆;其次阐述了生物礁具有的其它4个地球物理特征:较高的层度速、较的高波阻抗、无明显重力异常、低磁力异常.最后简要分析了西沙海域生物礁的油气地质条件,推测该区生物礁具有较好的油气资源前景,特别是临近生烃凹陷生物礁油气成藏条件最有利.

Reef is organic construction of carbonate rock generating from ree^building organisms, and mean while reef is significant target in oil and gas exploration. In the world scale, there are more than three hundred oil and gas fields which contain huge a mount of petroleum. Reef is famous for large a mount of production and reserve. In South China Sea （SCS）, based on studies during the last past fifty years, reef is convinced important exploration targets. There are lots of reef oil and gas fields which have been detected in there. Xisha offshore, lying in the northern of SCS, there initially experienced a rifting stage with post-rift relaxation during the Tertiary, the basin filled with shallow continental deposits which conclude both lacustrine and shallow marine sediments. The post-rift phase of evolution shows that Xisha offshore had gradually drowning of a carbonate platform, slope deposits, including reef and beach in vast area. The former studies have demonstrated an appropriate construction environment for reef since early Miocene, so it extremely developed on a large scale. Therefore, the studies of reef contain a significance meaning for exploration and economic. Based on the seismic data, the geophysical features are illustrated by the reality examples. The seismic data used in this paper are acquired by China National Petroleum Corporation from the pasl several years, including 2I） and 3D seismic data, and these data almost fully covered the whole area of Xisha offshore. The seismic profile quality is different with different areas and years, most of which are 2D multi-channel data and the acquisition density arranges from 4 X 8 km to 16 X ] 6 kin. The geophysical corresponding characteristics of reef are synthetically analyzed. First, the seismic reflection characteristics are introduced, including mound seismic reflection configuration, top interface＇s continuous reflection and strong amplitude, weakening continuous of bottom interface, unorganized internal reflection, lateral onlap, and draping overlying strata. Second, four geophysical characteristics are illustrated including high compression wave velocity, high compression wave impedance, none obvious abnormal gravity and abnormal low magnitude. I.ast, the hydrocarbon geological condition is analyzed, and the conclusion has been derived that this area is a good hydrocarbon prospect and important exploration target of deepwater in future.