Most custody gas chromatographs do not resolve every heavy hydrocarbon individually. Instead they group everything at hexane and above into a single lumped peak, often labelled C6+ or hexanes-plus, because separating out heptane, octane, and the rest would need a longer, slower analysis than a fast custody measurement allows. That lumped peak still has to be turned into a heating value and a density, and doing so means deciding how much of it is hexane, how much is heptane, and how much is octane and heavier. C6+ splitting is the fixed-ratio rule that makes that decision, and because the heavier fractions carry more energy per unit than the lighter ones, the ratio you choose changes the numbers the meter reports. This guide explains the common split ratios, why they shift computed BTU and density, and why the field GC and the SCADA recalculation engine must use exactly the same split.
C6+ Splitting in one line: C6+ splitting is the practice of dividing a single lumped hexanes-plus peak from a gas chromatograph into assumed proportions of hexane, heptane, and octanes-plus using a fixed ratio agreed in the contract, such as a 47/35/17 or 57/28/14 apportionment. The split is needed because a custody GC often reports everything from hexane up as one number, yet the physical properties of that group depend on how much of it is the heavier, higher-energy fractions. Because the assumed ratio changes the computed heating value and density of the stream, the same split factors must be configured identically in the field analyzer and in any SCADA or host recalculation so both arrive at the same energy figure.
A gas chromatograph separates a gas sample into its components by pushing it through a column where each compound travels at its own speed, and lighter molecules generally come off the column faster than heavier ones. Methane, ethane, propane, the butanes, and the pentanes are common and light enough that a fast custody analysis resolves them cleanly and reports each as its own mole percent. The hexanes and everything heavier are present in small amounts and take much longer to elute, so rather than wait for each one, many custody instruments backflush or group them and report the whole tail as a single hexanes-plus or C6+ number. That keeps the analysis short, which matters when the GC is cycling every few minutes to feed a custody meter.
The problem is that hexanes-plus is not one substance with one set of properties. It contains hexane isomers, heptanes, octanes, and heavier compounds, and each successive carbon number carries more energy and more mass than the one before it. A hexanes-plus fraction that is mostly hexane behaves very differently in a heating value and density calculation than one that is mostly octane and heavier, even at the same mole percent. So the single number the GC reports is not enough on its own to compute the stream's energy content; the calculation needs to know the internal makeup of that lumped group.
C6+ splitting resolves this by assuming a fixed internal composition for the lumped peak. Instead of measuring the exact breakdown, which the fast analysis did not capture, the calculation applies an agreed ratio that says, for example, that of all the moles in the C6+ group a certain fraction is hexane, a certain fraction is heptane, and the remainder is octanes-plus. Those individual pieces then each get their own physical properties from the reference tables, and the sum reconstructs a heating value and density for the group as if the components had been measured separately.
The split ratios in common use are simple sets of percentages that sum to one hundred. A 47/35/17 split assigns roughly forty-seven percent of the C6+ moles to hexane, thirty-five percent to heptane, and the remaining seventeen or eighteen percent to octanes-plus. A 57/28/14 split weights the group more heavily toward hexane, giving fifty-seven percent to hexane, twenty-eight percent to heptane, and fourteen or fifteen percent to octanes-plus. Industry guidance publishes several such standard ratios, and a gas contract will usually name the one that applies to a given delivery point so that both parties compute the same properties from the same lumped measurement.
The choice of split matters because heptane and octane carry more heating value and more mass per mole than hexane. A split that pushes more of the group toward the heavier end raises both the computed heating value contribution of the C6+ fraction and its contribution to the stream's density and relative gravity, while a split weighted toward hexane lowers both. Even though the C6+ fraction is a small mole percent of the whole stream, its per-mole energy is the largest of any component, so a shift in how it is characterized moves the total BTU per unit volume by a small but financially real amount over the volumes a custody point handles.
Because of this sensitivity, the split is not left to the analyzer's default; it is a contract term. The parties agree which ratio applies, and that ratio is then entered as configuration in the flow computer or GC that computes properties in the field and in any host system that recomputes them. If one side splits the C6+ as 47/35/17 and the other as 57/28/14, the two will disagree on heating value for the identical measured composition, and that disagreement shows up directly as an energy imbalance at settlement. Getting the split factors identical everywhere is therefore a prerequisite for the two calculations to reconcile.
In a typical custody setup the GC or its attached flow computer computes heating value and density on the spot from the measured composition, using its configured C6+ split, and reports both the raw mole percents and the derived properties. A SCADA or cloud host then collects those analyses and often recomputes properties itself, either to validate the field number, to apply the split the operator's contract specifies, or to feed downstream energy accounting. If the host applies a different split than the field device used, the recomputed heating value will not match what the meter used to convert volume to energy, and the two records will diverge for reasons that have nothing to do with the actual gas.
The clean discipline is to treat the split factors as configuration that must be provisioned identically in both places and verified whenever either is touched. When a GC is calibrated, a flow computer is replaced, or a contract split is changed, the same ratio has to be pushed to the field device and to the host recalculation, and the two should be reconciled by confirming they produce the same heating value from the same composition. A cloud monitoring platform such as Merobix is well placed to hold the authoritative split configuration for each delivery point, apply it consistently when it recomputes properties, and flag any analysis where the field-reported heating value and the host recomputation disagree beyond a small tolerance.
Trending the paired numbers over time is what turns this from a one-time setup into ongoing assurance. A host that stores both the field-computed and independently recomputed heating value for every analysis can show operators immediately if a split mismatch has crept in after a device swap, because the difference between the two curves will step at the moment the mismatch began. That makes the split a monitored quantity rather than a buried configuration item, so a misconfiguration is caught within a cycle or two rather than surfacing months later as an unexplained energy imbalance during a custody audit.
Heavy hydrocarbons are present in small amounts and take much longer to travel through the chromatograph column than the lighter components, so resolving each one individually would lengthen the analysis well beyond what a fast custody cycle allows. To keep the measurement quick enough to feed a custody meter every few minutes, many instruments group everything at hexane and above into a single hexanes-plus peak. The trade is that the internal makeup of that group is not measured, which is why a split is applied to characterize it.
Yes, because the hexanes-plus fraction carries the highest energy per mole of any component in the stream, so how it is characterized has an outsized effect on the total even though it is a small mole percent. A split weighted toward the heavier heptane and octane fractions raises the computed heating value and density, while a split weighted toward hexane lowers them. Over the large volumes a custody point handles, that difference becomes a real financial quantity, which is why the split is written into the contract.
They will compute different heating values and densities from the exact same measured composition, so the energy the meter records and the energy the host calculates will not agree. That divergence looks like an imbalance even though the gas and the raw analysis are identical, and it can trigger a custody dispute that is really just a configuration mismatch. The fix is to provision the same contract-specified split in both places and verify they produce matching properties from the same composition.
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