Stratigraphic distribution of the Codell Sandstone in the Denver Basin using wireline logs and core

Virginia Gent, R. Bottjer, M. Longman, J. Hagadorn
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Abstract

Core data from five key wells spanning the Denver Basin were tied to wireline log data and used to interpret the distribution of the Middle Turonian Codell Sandstone Member of the Carlile Formation across the Denver Basin. The character of the Codell’s upper contact is sharp with a localized top-down truncation across the basin, which is consistent with an associated unconformity surface. In contrast, the Codell’s lower contact varies from being gradational in most of the southern Denver Basin to being unconformable in the northern basin. Log correlations reveal that the Codell is absent within an elongate northeast-trending swath up to 125 miles wide in northeastern Colorado. This elongate gap is herein referred to as the ‘No Codell Zone’ abbreviated as NoCoZo. Hypotheses to explain the absence of the Codell Sandstone in the NoCoZo include a lateral facies change from sandstone to shale, non-deposition of Codell-equivalent sediments across this area, post-depositional erosion, or a combination of these processes. Correlation of wireline logs across the northern and southern limits of the NoCoZo, combined with outcrop and core observations, suggest top-down erosion of the Codell increasing into the NoCoZo. However, the overlying Fort Hays Limestone is laterally continuous and has a relatively consistent thickness across the NoCoZo, suggesting two tenable hypotheses: 1) The NoCoZo represents an area of post-Codell erosion due to short-lived growth of a broad, low relief uplift that was no longer active during Fort Hays deposition; or 2) A stepped sea level fall and forced regression resulting in non-deposition of the Codell over this broad swath. North of the NoCoZo, the Codell thickens northward to more than 40 ft into adjacent parts of Wyoming and Nebraska. In this northern area, the Codell has two main lithofacies in three laterally correlative zones, in ascending order: a lower bioturbated siltstone to very fine-grained sandstone ranging from 2 to 20 feet thick, a middle 2 to 10-foot thick laminated to bedded siltstone to fine-grained sandstone, and an upper 5 to 20-foot thick bioturbated siltstone to very fine-grained sandstone. Southeast of the NoCoZo the Codell thickens to as much as 80 feet in an east-trending belt from Pueblo, Colorado, into west central Kansas. The southern Codell can be divided into two coarsening upward parasequences, from a basal muddy coarse siltstones to very fine-grained sandstones. The siltstones and sandstones in the southern Codell are mostly bioturbated with locally developed bedded facies at the top.
利用电缆测井和岩心研究丹佛盆地Codell砂岩地层分布
研究人员将丹佛盆地5口关键井的岩心数据与电缆测井数据相结合,用于解释丹佛盆地卡莱尔组中Turonian Codell砂岩段的分布。科德尔上接触面特征鲜明,在盆地上有局部自上而下的截断,与不整合面相一致。相比之下,丹佛盆地南部大部分地区的科德尔下部接触为层序接触,而盆地北部则为不整合接触。测井相关性显示,在科罗拉多州东北部长达125英里宽的东北向狭长地带中没有科德尔。这个细长的间隙在这里被称为“无科德尔区”,缩写为NoCoZo。解释NoCoZo中Codell砂岩缺失的假说包括从砂岩到页岩的侧向相变化,该地区没有Codell等效沉积物的沉积,沉积后侵蚀或这些过程的组合。通过NoCoZo北部和南部边界的电缆测井对比,结合露头和岩心观测,表明Codell自上而下的侵蚀增加到NoCoZo。然而,上覆的Fort Hays石灰岩横向连续,在NoCoZo上具有相对一致的厚度,这提出了两个成立的假设:1)NoCoZo代表了后codell侵蚀区域,这是由于Fort Hays沉积期间不再活跃的广泛低起伏隆起的短暂生长造成的;或2)阶梯式海平面下降和强迫退潮,导致科德尔河在这片广阔地带上没有沉积。在NoCoZo以北,科德尔河向北加厚至40多英尺,进入怀俄明州和内布拉斯加州的邻近地区。在这个北部地区,Codell有两个主要的岩相,分布在三个横向相关的带中,依次为:下部的生物扰动粉砂岩到细粒砂岩,厚度从2到20英尺,中部的2到10英尺厚的层状到层状粉砂岩到细粒砂岩,上部的5到20英尺厚的生物扰动粉砂岩到非常细粒砂岩。在NoCoZo的东南部,从科罗拉多州的普韦布洛到堪萨斯州的中西部,科德尔河向东延伸,厚达80英尺。南科德尔可分为两个向上变粗的准层序,从基岩泥质粗粉砂岩到极细粒砂岩。南科德尔地区粉砂岩和砂岩以生物扰动为主,顶部为局部发育的层状相。
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