Catch Samples

Catch samples (also called cuttings samples, drill cuttings samples, or wellsite geological samples) are the fragments of formation rock produced by the drill bit during rotary drilling operations that are carried to surface by the circulating drilling fluid, collected at the shale shaker over a defined interval (typically every 1 to 5 metres), washed and sieved to remove drilling fluid, and examined by the wellsite geologist to determine the lithology, mineralogy, texture, porosity, hydrocarbon shows, and stratigraphic identity of the formations being drilled at depth, providing the primary real-time geological data stream that guides formation evaluation and well control decisions during the drilling of Western Canada Sedimentary Basin wells. The fundamental challenge of interpreting catch samples is the lag time between when the bit drills a formation and when the cuttings from that formation arrive at the shale shaker, because the cuttings must be transported upward through the annulus by the circulating mud, a journey that takes several minutes to over an hour depending on the annular velocity and the depth being drilled; the lag time (calculated as annular volume divided by pump rate, in strokes or minutes) must be accurately tracked by the mudlogger to ensure that the samples collected at the surface are correctly assigned to the depth at which they were cut by the bit rather than to the depth at the time of collection. In WCSB operations, catch samples are collected and described in real time by the mudlogging unit crew and the wellsite geologist, who wash each sample with water to remove the drilling mud, dry the sample on a paper towel, and examine it under a hand lens or binocular microscope to identify the rock type (sandstone, limestone, dolomite, shale, or other), estimate the grain size and sorting (for clastic rocks), assess the presence of visible porosity (intergranular, vuggy, or fracture), and detect hydrocarbon shows using a combination of visual fluorescence under ultraviolet light (produced oil fluoresces yellow to orange under UV; gas condensate fluoresces blue-white), olfactory detection of H2S or light hydrocarbon odors, and quantitative gas chromatograph analysis of the vapors evolved from freshly crushed or disaggregated samples in the mudlogger's gas extraction unit. The stratigraphic identity of each sample interval is established by comparing the lithological description and mineral content with the geological prognosis (the pre-drill prediction of formation tops and lithological sequence from offset well data), with formation tops called on the wellsite log in real time as distinctive marker beds are identified in the cuttings (the appearance of a distinctive dolomite horizon, a coal seam, a bentonite marker, or the lithological transition from shale to sandstone that marks the top of a reservoir unit) and confirmed by cross-referencing with the mudlogging gas show data, the drilling parameter trends (drilling rate changes at formation tops), and the directional survey data that places the cuttings sample in the correct depth-position in the wellbore. Catch sample quality in WCSB horizontal wells is significantly degraded compared to vertical wells because the extended lateral sections allow cuttings to settle and accumulate on the borehole low side rather than being efficiently transported to surface, creating a mixture of freshly cut material from the bit face with older material re-circulated from the cuttings bed in the lateral, requiring the wellsite geologist to apply additional interpretation skill to identify which components of a mixed sample represent the actual formation currently being drilled. Understanding catch sample collection procedures, lag time calculation and tracking, sample washing and preparation, lithological and show description techniques, UV fluorescence interpretation, gas chromatography integration, and the special challenges of cuttings interpretation in WCSB horizontal wells gives wellsite geologists, mudloggers, and formation evaluation engineers the practical foundation to generate accurate real-time geological logs that guide drilling decisions, formation top calls, casing point evaluations, and show assessments throughout the drilling of every WCSB well.

• Lag time calculation and tracking in WCSB drilling operations: Lag time (the time for cuttings to travel from bit to shale shaker) is calculated as annular volume (in litres or bbl) divided by pump output rate (litres per stroke times stroke rate), giving a lag time in strokes that the mudlogger converts to depth using the drill pipe tally. For a WCSB Montney well at 4,000 m MD with a 12-1/4 inch hole and 5-inch drill pipe, the annular volume is approximately 35 m3, and at a pump rate of 40 L/s the lag time is 875 seconds (approximately 14.5 minutes). The mudlogger maintains a real-time lag counter (often a stroke counter) and updates the catch sample depth assignments continuously as the bit advances, ensuring that a casing point or show call is made at the correct depth.
• UV fluorescence show evaluation in WCSB reservoir intervals: Hydrocarbon shows in WCSB catch samples are evaluated under long-wave ultraviolet light (365 nm wavelength) in a darkened sample description box. Crude oil shows fluoresce yellow, orange, or brown depending on API gravity and composition; gas condensate fluoresces blue-white; dead oil (oxidized or biodegraded) shows dull brown fluorescence; water-wet sand shows no fluorescence. A cut test (applying a drop of organic solvent such as chloroform or trichloroethylene to a crushed sample) confirms a hydrocarbon show by releasing an odor of dissolved hydrocarbons and producing a fluorescent oil ring on filter paper as the solvent evaporates, distinguishing residual oil in the rock pores from drilling mud fluorescence artifacts.
• Mudlogging gas chromatography for show quantification: Continuous gas chromatography in the mudlogging unit measures the concentration of C1 (methane) through C5 (pentane) hydrocarbon components in the gas stream extracted from the return drilling mud by a gas trap on the mud return line. The chromatograph output (plotted as total gas units and component ratios on the mudlog) identifies show intervals (elevated total gas), distinguishes oil-associated shows from gas shows (higher C2-C5 ratios in oil-associated gas vs. predominantly C1 in gas shows), and enables the Pixler ratio plot (C1/C2 versus C1/C3) that classifies show gas composition as oil zone, oil with gas cap, gas zone, or non-commercial shows. These data are integrated with the physical sample fluorescence to characterize each WCSB show interval before testing or wireline logging decisions are made.
• Formation top identification from WCSB catch samples: WCSB wellsite geologists call formation tops in real time by matching distinctive lithological signatures in the catch samples with the geological prognosis derived from offset wells and regional stratigraphy. The Cardium Formation top is called when glauconitic sandstone appears in samples after drilling through the Wapiabi shale; the Viking top is called at the first appearance of coarse-grained clean sandstone grains after the Colorado shale; the Montney top is identified by the appearance of fine-grained siliceous siltstone with low porosity after the Triassic Doig Formation. Correct formation top identification in real time enables timely decisions about casing points, directional drilling landing targets, and formation evaluation program initiation.
• Sample contamination and interpretation challenges in WCSB drilling: Several contamination sources degrade catch sample quality in WCSB operations: cavings (material that sloughs from the borehole wall above the bit and mixes with fresh cuttings at the surface, identifiable by anomalously large fragment size and angular shape inconsistent with bit grinding); recycled cuttings (material that has been re-ground by the bit after settling in the hole); drilling fluid additives (barite, calcium carbonate, and polymer beads that create non-formation grains in the sample); and cement residue from previous casing shoe drill-outs. Wellsite geologists use grain size analysis, UV fluorescence, and comparison with the drilling parameter log to distinguish genuine formation cuttings from contaminants in the described sample.

Formation Top Call from Catch Samples Triggering Casing Point Decision on a WCSB Montney Well

While drilling the intermediate hole section of a northeast British Columbia Montney horizontal well, the wellsite geologist monitoring catch samples at 2,285 m MD (lag-corrected) identified a sudden change from gray marine shale (Triassic Doig Formation) to pale gray siliceous siltstone with faint UV fluorescence and elevated total gas (background of 45 units increasing to 280 units over 4 metres). The description matched the Montney Formation top prognosis at 2,290 m MD from the offset well geological prognosis. Simultaneously, the MWD pore pressure indicators showed a 0.08 sg increase in ECD compared to the background trend, consistent with entering the overpressured Montney. The wellsite geologist radioed the Montney top call to the drilling engineer on call. The engineer reviewed the lag-corrected sample depth against the prognosis (actual top 5 m shallower than prognosis), confirmed the mud weight window had narrowed from 0.14 sg to 0.06 sg margin above the new pore pressure indicator, and directed the driller to circulate bottoms-up and prepare to set the intermediate casing at 2,290 m. The casing was set that afternoon; the FIT after drill-out confirmed 1.72 sg EMW at the shoe, providing adequate margin to drill the Montney lateral with 1.58 sg mud. The real-time catch sample top call prevented drilling 30 to 50 metres into the overpressured Montney with insufficient mud weight, avoiding a probable well control event.

Related Terms

Mudlogging is the wellsite service that collects, describes, and records catch samples continuously during WCSB drilling operations, integrating the lithological and show data from cuttings with the gas chromatography measurements and drilling parameter trends to produce the mudlog that is the primary real-time formation evaluation record for every well. Shale shaker is the vibrating screen device at the wellsite that separates drill cuttings from the returned drilling fluid as the first step in the solids control system, with the screened cuttings collected by the mudlogger for lag-time-corrected sample description at the point where they discharge from the shaker screens. Lag time is the calculated delay between when the drill bit cuts a formation and when the cuttings from that formation arrive at the shale shaker, equal to the annular volume divided by the circulation rate; accurate lag time tracking is essential for correctly assigning each catch sample to its true drilled depth on the mudlog rather than to the surface collection time. UV fluorescence is the show detection technique applied to WCSB catch samples under ultraviolet light to identify oil staining in the pores of the cuttings fragments, with the fluorescence color and intensity providing an initial qualitative assessment of the hydrocarbon type and quantity before wireline logging or formation testing is conducted. Formation top identification from catch samples is the primary stratigraphic interpretation task of the WCSB wellsite geologist, calling the depth at which each geological formation is first encountered based on lithological changes in the cuttings and confirming the call against the mudlogging gas show data, drilling parameter trends, and the pre-drill geological prognosis before reporting the top to the drilling engineer for casing point and directional drilling decisions.