| 摘要: | 台灣於中期漸新世至晚期中新世時期為被動大陸邊緣，但仍有局部張裂活動，可能有小型張裂盆地發育。為了建立完整層序地層架構，本研究利用台灣中油公司提供之台灣西南部陸域鑽井資料，使用B斷層及義竹斷層鄰近區域共11口井進行沉積環境與層序地層分析。主要方法是利用伽瑪射線、自然電位、聲波井測以及電阻率等電測曲線的變化，劃分不同特徵的電測相，再佐以岩屑紀錄中的特殊沉積物特徵，進行沉積環境分析。最後再依據其垂直的環境變化劃分出不同的次層序來建立層序地層。
本研究共分類出8種沉積相及8種次層序，研究結果顯示八掌溪砂岩及其同期地層以濱海相環境為主；碧靈頁岩至打鹿頁岩時期則是以濱面至遠濱相環境為主；觀音山砂岩至南莊層時期則是以受潮汐以及波浪影響的堰洲島、潟湖、潮坪等環境與濱面相為主。根據次層序的垂直變化，本研究參考前人之命名及劃分方法，將八掌溪砂岩(及其相當地層)至南莊層劃分為K0、M0、M2、M4、M6、M8、M10、M12+M14、M16、M18、M20、M22、M24、M26等，共14個層序。
根據鑽井之地層厚度及層序變化發現，八掌溪砂岩及其相當地層於義竹斷層以北地區厚度約為60~100公尺，以南最厚可達到約465公尺；南莊層則於JY-1井達到約1200公尺，推測其受到斷層影響。因此可依據層序地層厚度變化，將中新世地層分為三個發育時期 (1) K0、M0及M2層序(30~21 Ma)：處於張裂階段，義竹斷層開始活動並形成地塹，堆積八掌溪砂岩及其相當地層或更老地層，(2) M4~M18層序(21~12.5 Ma)：盆地基盤呈穩定的熱沉降，各地層側向厚度變化不大，無明顯構造活動，堆積碧靈頁岩至打鹿頁岩，(3) M20~M26層序(12.5~6.5 Ma)： B斷層及其南部斷層開始發育，B斷層以北形成高區，義竹斷層以南則形成地塹，此時可對應於觀音山砂岩及南莊層。;Taiwan had been a passive continental margin from the middle Oligocene to late Miocene. There were two local extensional events, leading to the formation of small rifted basins during the passive-margin stage. In order to establish a comprehensive sequence stratigraphic framework, we use well-log data of 11 boreholes near the B Fault and Yichu Fault in SW Taiwan for paleo-environmental and sequence stratigraphic analyses. The well data was provided by CPC Corporation.
Eight sedimentary facies and 8 types of parasequences are identified. Our results shows that Pachangchi Sandstone was accumulated in the shoreface environment. The overlying formations (i.e., Piling Shale to Talu Shale) exhibit offshore to shoreface environments. During the deposition of the Kuanyinshan Sandstone and the Nanchuang Formation, the environment changed into a tide-and-wave-dominated environment, consisting of tidal deposits, barrier complexes and shoreface deposits. For the sequence stratigraphic analysis, the Miocene strata is divided into 14 sequences, namely, K0, M0, M2, M4, M6, M8, M10, M12+M14, M16, M18, M20, M22, M24 and M26, in an ascending order.
According to borehole stratigraphic correlations and sequence stratigraphic analyses, we found that the thickness of the Pachangchi Sandstone and its equivalents at KTL-3 well is quite thick, up to around 400 m. While the thickness of the Pachangchi Sandstone in the rest of studied borehole is only 60 m to 100 m. Similarly, the thickness of Nanchuang Formation attains up to about 1200 m at HSY-1 well. Base on this observation and previous study, we divided the tectonic activity in this region into 3 stages. (1) K0, M0, M2 sequences (early post break up, 30~21 Ma): Yichu Fault became active and created a half graben as observed at KTL-3 well. Sequences of K0, M0 and M2 were deposited. (2) M4~M18 sequence (21~12.5 Ma, thermal subsidence): Thickness of each formation and sequence remains consistent, no major tectonic activities in this period. Sequences of M4~M18 were accumulated during this period (3) M20~M26 sequences (12.5~6.5 Ma, renewed extension): Yichu Fault and B Fault became active, leading to the formation of a graben as seen at the JY-1 well, while the area near northern B Fault became an uplifted platform. Sequences of M20~M26 were deposited. |
